Fixed #23244: Error altering FK to non-FK in migrations
Moved index dropping after FK dropping to please MySQL and fix test
Fixed #23226: Model options appearing as bytes type in migrations
Removed a doc reference to the deprecated `mimetype` kwarg.
Fixed #23252: Call out removed features in release notes.

Also added StrAndUnicode mixin note to deprecation plan as it was
missing.
Added a missing unicode_literals that caused a test failure after refs #23226.
Use smart_text rather than naive forcing-to-unicode for state reading
Fixed flake8 warning.
Fixed #22982 -- Added GenericRelatedObjectManager.__str__. to prevent crash.

Thanks bendavis78 for the report.

Forwardport of 29585e9b6a from stable/1.7.x
Don't treat .pyc/pyo files as migrations. Refs #23237 among others.