provision: Check for old Ubuntu or Python before starting Python.

This unblocks us from being able to use Python 3.6 syntax in
provision.py and its dependencies.

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

.circleci/config.yml

@@ -154,8 +154,8 @@ aliases:
     run:
       name: check tools/provision error message on xenial
       command: |
-        ! tools/provision > >(tee provision.out)
-        grep -Fqx 'CRITICAL:root:Unsupported platform: ubuntu 16.04' provision.out
+        ! tools/provision 2> >(tee provision.err >&2)
+        grep -Fqx 'Error: ubuntu 16.04 is no longer a supported platform for Zulip.' provision.err
 
   - &check_xenial_upgrade_error
     run:

tools/lib/provision.py

@@ -104,12 +104,6 @@ elif vendor == "centos" and os_version == "7":
     POSTGRES_VERSION = "10"
 else:
     logging.critical("Unsupported platform: %s %s", vendor, os_version)
-    if vendor == 'ubuntu' and os_version == '14.04':
-        print()
-        print("Ubuntu Trusty reached end-of-life upstream and is no longer a supported platform for Zulip")
-        if os.path.exists('/home/vagrant'):
-            print("To upgrade, run `vagrant destroy`, and then recreate the Vagrant guest.\n")
-            print("See: https://zulip.readthedocs.io/en/latest/development/setup-vagrant.html")
     sys.exit(1)
 
 VENV_DEPENDENCIES = get_venv_dependencies(vendor, os_version)

tools/provision

@@ -10,6 +10,27 @@ if [ "$EUID" -eq 0 ]; then
     exit 1
 fi
 
+os="$(. /etc/os-release && echo "$ID $VERSION_ID")"
+case "$os" in
+    'ubuntu 14.04' | 'ubuntu 16.04')
+        echo "Error: $os is no longer a supported platform for Zulip." >&2
+        if [ -e /home/vagrant ]; then
+            # shellcheck disable=SC2016
+            echo 'To upgrade, run `vagrant destroy`, and then recreate the Vagrant guest.' >&2
+            echo 'See: https://zulip.readthedocs.io/en/latest/development/setup-vagrant.html' >&2
+        fi
+        exit 1
+        ;;
+esac
+
+python_version="$(python3 --version)"
+case "$python_version" in
+    Python\ 3.[0-5].*)
+        echo 'Error: Zulip requires an OS with Python 3.6 or later.' >&2
+        exit 1
+        ;;
+esac
+
 FAIL='\033[91m'
 WARNING='\033[93m'
 ENDC='\033[0m'