Bug 1348375 - Re-enable flake8 rule F401

Since it's actually needed to catch unused imports, albeit it's a bit
more strict than it used to be, so required some fix-ups before it could
be re-enabled.

setup.cfg

@@ -14,10 +14,9 @@ max-line-length = 140
 # pycodestyle's config (eg autopep8) so we can't just define everything under [flake8].
 exclude = .git,__pycache__,.vagrant,node_modules
 # The ignore list for pycodestyle above, plus our own PyFlakes additions:
-# F401: module imported but unused
 # F403: 'from module import *' used; unable to detect undefined names
 # F405: name may be undefined, or defined from star imports: module
-ignore = E121,E123,E126,E129,E226,E24,E704,W503,E501,F401,F403,F405
+ignore = E121,E123,E126,E129,E226,E24,E704,W503,E501,F403,F405
 max-line-length = 140
 
 [isort]