Add note why E712 is ignored

According to the commit message in
560e5cd84ef2c1a6427c78d35eedc66445e1f77f we ignore E712 because '
With this change we ignore E712 since it is normal to use
"column == True" in sqlalchemy.'

Add note in tox.ini explaining this.

Change-Id: I393e13b1430aae46f958311875678bd99e17b13c
Add expected_errors for extension certificates v3

Partially implements: bp v3-api-extension-versioning

Change-Id: I3610cdbcca4f465e071e54da2965f4016acea9c5
Remove unused variable

Change-Id: I000d6255c5ada5f41dc8e8f9783dea11c1acfca8
Merge "Add note why E712 is ignored"
Merge "Remove unused variable"
Merge "Add expected_errors for extension certificates v3"