[1.7.x] Fixed #23244: Error altering FK to non-FK in migrations
[1.7.x] Moved index dropping after FK dropping to please MySQL and fix test
[1.7.x] Fixed #23226: Model options appearing as bytes type in migrations
[1.7.x] Removed a doc reference to the deprecated `mimetype` kwarg.

Backport of 61ed959235 from master
[1.7.x] Fixed #23252: Call out removed features in release notes.

Also added StrAndUnicode mixin note to deprecation plan as it was
missing.
[1.7.x] Added a missing unicode_literals that caused a test failure after refs #23226.

Backport of 57b60f9f93 from master
[1.7.x] Use smart_text rather than naive forcing-to-unicode for state reading
[1.7.x] Fixed flake8 warning.

Backport of f9f9f3ad60 from master
[1.7.x] Fixed #22982 -- Added GenericRelatedObjectManager.__str__. to prevent crash.

Thanks bendavis78 for the report.
[1.7.x] Don't treat .pyc/pyo files as migrations. Refs #23237 among others.