use flake8 to lint for a few things black doesn't catch

black does *not* warn about missing or extraneous imports,
so let's bring back flake8 in our linting to check for them

awx/main/utils/common.py

@@ -7,7 +7,6 @@ import yaml
 import logging
 import os
 import re
-import subprocess
 import stat
 import urllib.parse
 import threading