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
Merge "Add note why E712 is ignored"