[1.7.x] Revert "Fixed #23455 -- Forced related_name to be a unicode string during deconstruction."

This reverts commit 45bd7b3bd9008941580c100b9fc7361e3ff3ff0d.

This is a backport of 8aaf51f94c70e3cfcd2c75a0be1b6f55049d82d8 from master.
[1.7.x] Fixed #23982 -- Added doc note on generating Python 2/3 cross-compatible migrations.

Thanks Luke Plant for the report, and Tim Graham, Simon Charette, and Markus
Holtermann for review and discussion.

This is a backport of d4bdddeefe35214f194cb85e40dbb761c206e7fa from master.
[1.7.x] Fixed #23455 -- Accept either bytes or text for related_name, convert to text.

Backport of c72eb80d114fb5d90bd21b5549e8abd0bbd17f99 from master.