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.
Fixed table cleanup in GIS migration tests
Fixed a remote code execution vulnerabilty in URL reversing.

Thanks Benjamin Bach for the report and initial patch.

This is a security fix; disclosure to follow shortly.
Prevented leaking the CSRF token through caching.

This is a security fix. Disclosure will follow shortly.
Fixed queries that may return unexpected results on MySQL due to typecasting.

This is a security fix; disclosure to follow shortly.