Lint the contrib/ directory in CI and linting scripts, add synctl to linting script (#7914)

Run `isort`, `flake8` and `black` over the `contrib/` directory and `synctl` script. The latter was already being done in CI, but now the linting script does it too.

Fixes https://github.com/matrix-org/synapse/issues/7910

contrib/scripts/kick_users.py

@@ -1,10 +1,12 @@
 #!/usr/bin/env python
 from __future__ import print_function
-from argparse import ArgumentParser
+
 import json
-import requests
 import sys
 import urllib
+from argparse import ArgumentParser
+
+import requests
 
 try:
     raw_input