lint: Run isort as a linter.

With a change to increase the line length to 100 characters by
tabbott.

Fixes #2665.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

.isort.cfg

@@ -3,4 +3,4 @@ multi_line_output = 3
 known_third_party = netifaces
 include_trailing_comma = True
 use_parentheses = True
-line_length = 88
+line_length = 100

tools/lint

@@ -89,6 +89,9 @@ def run() -> None:
     linter_config.external_linter('gitlint', ['tools/commit-message-lint'],
                                   description="Checks commit messages for common formatting errors."
                                   "(config: .gitlint)")
+    linter_config.external_linter('isort', ['tools/run-isort'], ['py'],
+                                  description="Sorts Python import statements",
+                                  fix_arg='--fix')
 
     semgrep_command = ["semgrep", "--config=./tools/semgrep.yml", "--error",
                        # This option is dangerous in the context of running

tools/run-isort

@@ -0,0 +1,8 @@
+#!/bin/sh
+set -eu
+if [ "$1" = "--fix" ]; then
+    shift
+    exec isort "$@"
+else
+    exec isort --check-only --diff "$@"
+fi