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>

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'