Fixed #22473 -- Emphasized that Model.clean() is not called from Model.save().
Fixed #22458 -- Added a note about MySQL utf8_unicode_ci collation

Thanks tobami at gmail.com for the report.
Moved RemoteUserBackend documentation to reference guide.
Fixed #22397 -- Issues removing M2M field with explicit through model

Changed the migration autodetector to remove models last so that FK
and M2M fields will not be left as dangling references. Added a check
in the migration state renderer to error out in the presence of
dangling references instead of leaving them as strings. Fixed a bug
in the sqlite backend to handle the deletion of M2M fields with
"through" models properly (i.e., do nothing successfully).

Thanks to melinath for report, loic for tests and andrewgodwin and
charettes for assistance with architecture.
Fixed #22417 -- Added additional documentation for refs #16969.

Thanks Jon Foster for the report.
Changed paths in docs that referred to Python 2.
Fixed flake8 errors.
Removed old test runner test cases.

These files were added as part of #12658, which pertained to the old django.test.simple test runner. No tests call them anymore.
Fixed #22402 -- Consolidated model_inheritance tests.

The model_inheritance_same_model_name tests couldn't be run without the
model_inheritance tests. Make the problem go away by merging them.

Thanks timo for the report.
Further consolidated the model_inheritance tests.
Consolidated get_or_create tests.
Consolidated expressions tests.
Appeased flake8 2.1.0.
Fixed monkeypatching in a staticfiles test.
Fixed migrate when called inside a transaction.

This is useful for tests manually calling migrate inside a testcase,
for normal usage this should make no difference, since there is no
surrounding transaction after all. If there is one we still try to
leave the transaction in a useable state (for postgres at least).

If this commit turns out to be causing issues, settings savepoint=False
is probably the right fix :)
Merge pull request #2592 from prestontimmons/remove-test-runner-cases

Removed old test runner test cases.
Corrected the section identifier for MySQL unicode reference.
Fixed adding new GIS column in PostGIS 1.x migration

Refs #22451.
Fixed removal of GIS column in PostGIS 1.x migration

Refs #22481.
Fixed PostGIS remove_field implementation

Fixed bad code introduced in 2f9d1576e8.