Cleanup the flake8 section of tox.ini

* Stop ignoring F403 as we have no cases of it anyway
* Cleanup the comments to make it clear what is ignored on purpose and
  what is ignored until we clean up the code.

Change-Id: I2500e2db70b9c71fbed781d8f7b7c80eee45140f
Merge "Cleanup the flake8 section of tox.ini"