diff --git a/requirements/dev.in b/requirements/dev.in
index 75a1f7a252..cfc2c19b08 100644
--- a/requirements/dev.in
+++ b/requirements/dev.in
@@ -32,6 +32,9 @@ line-profiler
 # for pep8 linter
 pycodestyle
 
+# Python reformatter
+black
+
 # Needed to run pyflakes linter
 pyflakes
 
diff --git a/requirements/dev.txt b/requirements/dev.txt
index 568275545d..c0c4c65b97 100644
--- a/requirements/dev.txt
+++ b/requirements/dev.txt
@@ -21,6 +21,10 @@ apns2==0.7.2 \
     --hash=sha256:4f2dae8c608961d1768f734acb1d0809a60ac71a0cdcca60f46529b73f20fb34 \
     --hash=sha256:f64a50181d0206a02943c835814a34fc1b1e12914931b74269a0f0fb4f39fd45
     # via -r requirements/common.in
+appdirs==1.4.4 \
+    --hash=sha256:7d5d0167b2b1ba821647616af46a749d1c653740dd0d2415100fe26e27afdf41 \
+    --hash=sha256:a841dacd6b99318a741b166adb07e19ee71a274450e68237b4650ca1055ab128
+    # via black
 argon2-cffi==20.1.0 \
     --hash=sha256:05a8ac07c7026542377e38389638a8a1e9b78f1cd8439cd7493b39f08dd75fbf \
     --hash=sha256:0bf066bc049332489bb2d75f69216416329d9dc65deee127152caeb16e5ce7d5 \
@@ -81,6 +85,9 @@ beautifulsoup4==4.9.3 \
     #   -r requirements/common.in
     #   pyoembed
     #   zulip-bots
+black==20.8b1 \
+    --hash=sha256:1c02557aa099101b9d21496f8a914e9ed2222ef70336404eeeac8edba836fbea
+    # via -r requirements/dev.in
 boto3==1.17.5 \
     --hash=sha256:0d126d429a8a9ea2c8409a1ee36ce5a34e11db788d7730a714016264f10bbb89 \
     --hash=sha256:d6aafb804fca2b67c65dda78ad8b4afed901e004071208b84c804d345ad9ebba
@@ -199,6 +206,7 @@ click==7.1.2 \
     --hash=sha256:d2b5255c7c6349bc1bd1e59e08cd12acbbd63ce649f2588755783aa94dfb6b1a \
     --hash=sha256:dacca89f4bfadd5de3d7489b7c8a566eee0d3676333fbb50030263894c38c0dc
     # via
+    #   black
     #   click-log
     #   fb-sapp
     #   flask
@@ -310,6 +318,7 @@ dataclasses==0.8 ; python_version < "3.7" \
     --hash=sha256:8479067f342acf957dc82ec415d355ab5edb7e7646b90dc6e2fd1d96ad084c97
     # via
     #   -r requirements/common.in
+    #   black
     #   dataclasses-json
     #   libcst
     #   pyre-check
@@ -830,6 +839,7 @@ mypy-extensions==0.4.3 \
     --hash=sha256:090fedd75945a69ae91ce1303b5824f428daf5a028d2f6ab8a299250a846f15d \
     --hash=sha256:2d82818f5bb3e369420cb3c4060a7970edba416647068eb4c5343488a6c604a8
     # via
+    #   black
     #   mypy
     #   typing-inspect
 mypy==0.800 \
@@ -919,6 +929,10 @@ parso==0.8.1 \
     --hash=sha256:15b00182f472319383252c18d5913b69269590616c947747bc50bf4ac768f410 \
     --hash=sha256:8519430ad07087d4c997fda3a7918f7cfa27cb58972a8c89c2a0295a1c940e9e
     # via jedi
+pathspec==0.8.1 \
+    --hash=sha256:86379d6b86d75816baba717e64b1a3a3469deb93bb76d613c9ce79edc5cb68fd \
+    --hash=sha256:aa0cb481c4041bf52ffa7b0d8fa6cd3e88a2ca4879c533c9153882ee2556790d
+    # via black
 pexpect==4.8.0 \
     --hash=sha256:0b48a55dcb3c05f3329815901ea4fc1537514d6ba867a152b581d69ae3710937 \
     --hash=sha256:fc65a43959d153d0114afe13997d439c22823a27cefceb5ff35c2178c6784c0c
@@ -1285,7 +1299,9 @@ regex==2020.11.13 \
     --hash=sha256:e3faaf10a0d1e8e23a9b51d1900b72e1635c2d5b0e1bea1c18022486a8e2e52d \
     --hash=sha256:f7d29a6fc4760300f86ae329e3b6ca28ea9c20823df123a2ea8693e967b29917 \
     --hash=sha256:f8f295db00ef5f8bae530fc39af0b40486ca6068733fb860b42115052206466f
-    # via talon
+    # via
+    #   black
+    #   talon
 requests-oauthlib==1.3.0 \
     --hash=sha256:7f71572defaecd16372f9006f33c2ec8c077c3cfa6f5911a9a90202beb513f3d \
     --hash=sha256:b4261601a71fd721a8bd6d7aa1cc1d6a8a93b4a9f5e96626f8e4d91e8beeaa6a
@@ -1570,6 +1586,10 @@ tlds==2021020900 \
     --hash=sha256:9774b3df448cd880870c5fc409c91b7b0b17d2b1d433a7fab63be5c0facc25e9 \
     --hash=sha256:bfd0d63f863b150e451717b35f6cea4621264334e6dd000075edf135e35e0c2a
     # via -r requirements/common.in
+toml==0.10.2 \
+    --hash=sha256:806143ae5bfb6a3c6e736a764057db0e6a0e05e338b5630894a5f779cabb4f9b \
+    --hash=sha256:b3bda1d108d5dd99f4a20d24d9c348e91c4db7ab1b749200bded2f839ccbe68f
+    # via black
 tornado==4.5.3 \
     --hash=sha256:5ef073ac6180038ccf99411fe05ae9aafb675952a2c8db60592d5daf8401f803 \
     --hash=sha256:6d14e47eab0e15799cf3cdcc86b0b98279da68522caace2bd7ce644287685f0a \
@@ -1648,13 +1668,16 @@ typed-ast==1.4.2 \
     --hash=sha256:d175297e9533d8d37437abc14e8a83cbc68af93cc9c1c59c2c292ec59a0697a3 \
     --hash=sha256:d746a437cdbca200622385305aedd9aef68e8a645e385cc483bdc5e488f07166 \
     --hash=sha256:e683e409e5c45d5c9082dc1daf13f6374300806240719f95dc783d1fc942af10
-    # via mypy
+    # via
+    #   black
+    #   mypy
 typing-extensions==3.7.4.3 \
     --hash=sha256:7cb407020f00f7bfc3cb3e7881628838e69d8f3fcab2f64742a5e76b2f841918 \
     --hash=sha256:99d4073b617d30288f569d3f13d2bd7548c3a7e4c8de87db09a9d29bb3a4a60c \
     --hash=sha256:dafc7639cde7f1b6e1acc0f457842a83e722ccca8eef5270af2d74792619a89f
     # via
     #   -r requirements/common.in
+    #   black
     #   fb-sapp
     #   importlib-metadata
     #   libcst
diff --git a/setup.cfg b/setup.cfg
index 3ac6beb509..450a8d4885 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -2,74 +2,26 @@
 ignore =
     # Each of these rules are ignored for the explained reason.
 
-    # "whitespace before ':'"
-    # Black disagrees with this.
-    E203,
-
-    # "multiple spaces before operator"
-    # There are several typos here, but also several instances that are
-    # being used for alignment in dict keys/values using the `dict`
-    # constructor. We could fix the alignment cases by switching to the `{}`
-    # constructor, but it makes fixing this rule a little less
-    # straightforward.
-    E221,
-
-    # 'missing whitespace around arithmetic operator'
-    # This should possibly be cleaned up, though changing some of
-    # these may make the code less readable.
-    E226,
-
-    # New rules in pycodestyle 2.4.0 that we haven't decided whether to comply with yet
-    E252, W504,
-
-    # "multiple spaces after ':'"
-    # This is the `{}` analogue of E221, and these are similarly being used
-    # for alignment.
-    E241,
-
-    # "unexpected spaces around keyword / parameter equals"
-    # Many of these should be fixed, but many are also being used for
-    # alignment/making the code easier to read.
-    E251,
-
-    # "block comment should start with '#'"
-    # These serve to show which lines should be changed in files customized
-    # by the user. We could probably resolve one of E265 or E266 by
-    # standardizing on a single style for lines that the user might want to
-    # change.
-    E265,
-
-    # "too many leading '#' for block comment"
-    # Most of these are there for valid reasons.
-    E266,
-
-    # "expected 2 blank lines after class or function definition"
-    # Zulip only uses 1 blank line after class/function
-    # definitions; the PEP-8 recommendation results in super sparse code.
-    E302, E305,
+    # All formatting is handled by Black.
+    E1,
+    E2,
+    E3,
+    E401,
+    E5,
+    E701,
+    E702,
+    E703,
+    E704,
+    W1,
+    W2,
+    W3,
+    W5,
 
     # "module level import not at top of file"
     # Most of these are there for valid reasons, though there might be a
     # few that could be eliminated.
     E402,
 
-    # "line too long"
-    # Zulip is a bit less strict about line length, and has its
-    # own check for this (see max_length)
-    E501,
-
     # "do not assign a lambda expression, use a def"
     # Fixing these would probably reduce readability in most cases.
     E731,
-
-    # "line break before binary operator"
-    # This is a bug in the `pep8`/`pycodestyle` tool -- it's completely backward.
-    # See https://github.com/PyCQA/pycodestyle/issues/498 .
-    W503,
-
-    # This number will probably be used for the corrected, inverse version of
-    # W503 when that's added: https://github.com/PyCQA/pycodestyle/pull/502
-    # Once that fix lands and we update to a version of pycodestyle that has it,
-    # we'll want the rule; but we might have to briefly ignore it while we fix
-    # existing code.
-    # W504,
diff --git a/tools/lint b/tools/lint
index 49eb17ab80..7c639df415 100755
--- a/tools/lint
+++ b/tools/lint
@@ -2,6 +2,7 @@
 import argparse
 import os
 import random
+import re
 import sys
 
 # check for the venv
@@ -145,6 +146,15 @@ def run() -> None:
         fix_arg=["--write"],
         description="Formats CSS, JavaScript, YAML",
     )
+    linter_config.external_linter(
+        "black",
+        ["black"],
+        ["py"],
+        description="Reformats Python code",
+        check_arg=["--check"],
+        suppress_line=lambda line: line == "All done! ✨ 🍰 ✨\n"
+        or re.fullmatch(r"\d+ files? would be left unchanged\.\n", line) is not None,
+    )
 
     semgrep_command = [
         "semgrep",
diff --git a/version.py b/version.py
index 8f06b277be..deeb5c1148 100644
--- a/version.py
+++ b/version.py
@@ -45,4 +45,4 @@ API_FEATURE_LEVEL = 38
 #   historical commits sharing the same major version, in which case a
 #   minor version bump suffices.
 
-PROVISION_VERSION = "126.2"
+PROVISION_VERSION = "126.3"