Thanks blaze33 for the patch.

Thanks Roman Gladkov and Zbigniew Siciarz.

Thanks Kronuz and regebro.

Thanks Claude and Julien for the review.

Thanks Erik Romijn.

Fixed #20043 -- Removed unused BLANK_CHOICE_NONE

The backwards-incompatible changes section wasn't structured in sections
like it is in release notes for previous versions.

Some alternative implementations don't have them.

Closes #19944.

The AssertNumQueriesContext didn't reset connection's use_debug_cursor
in case there was an exception. This resulted in leaking query strings
into connection.queries. Maximum memory use pre-patch was around 700MB,
post-patch it is around 200MB for Django's test suite.

Fix #20058: Make compilemessages use stdout instead of stderr.

Fix #20054: Removed links to modwsgi.org.

Fix 20061: remove out of date comment.

Thanks slinkp for the report and the initial patch.

Fixed #20053 -- Fix `index_together` documentation

Fixed an erroneous import in example code of docs (class-based-views/intro).

The original problem was that when filtering two levels up in
inheritance chain, Django optimized the join generation so that the
middle model was skipped. But then Django generated joins from top
to middle to bottom for SELECT clause, and thus there was one extra
join (top->middle->bottom + top -> bottom).

This case is fixed in master as the filtering optimization is gone.
This has the side effect that in some cases there is still extra join
if the SELECT clause doesn't contain anything from middle or bottom.

When the 'invalid' error message is set at field level, it masks
the error message raised by the validator, if any.

Thanks Florian Apolloner for the report and the review.
Also fixes #19643.

Model.save() will use UPDATE - if not updated - INSERT instead of
SELECT - if found UPDATE else INSERT. This should save a query when
updating, but will cost a little when inserting model with PK set.

Also fixed #17341 -- made sure .save() commits transactions only after
the whole model has been saved. This wasn't the case in model
inheritance situations.

The save_base implementation was refactored into multiple methods.
A typical chain for inherited save is:
save_base()
    _save_parents(self)
        for each parent:
            _save_parents(parent)
            _save_table(parent)
    _save_table(self)

Thanks Ram Rachum for the report.