Fixed #22299 -- Cleanup wizard temp files when possible.

Thanks to @erikr for the review.
Fixed PostGIS test failures.
Fixed #22291 -- Avoided shadowing deadlock exceptions on MySQL.

Thanks err for the report.
Fixed #22317 -- Fixed documented handler400 default value
Fixed #22318 -- Added Form.has_error() to easily check if a given error has happened.
Fixed #22265 -- Clarfied consistent behavior of migrations.

Thanks aruseni for the report.
Updated six to 1.6.1.
Fixed #17922 -- Added required_css_class to form label.

Thanks hanson2010 for the suggestion.
Fixed #21665 -- Documented that changes in assertRedirects may be required given new URL escaping behavior.

Thank pdc for the report.
Fixed #22247 -- Replaced "upstream" with "downstream" in cache docs.

Thanks valgarv at gmx.net for the report.
Fixed #22048 - Enhanced docs to cover nonexistent one-to-one relationships.

Thanks EvilDMP for the suggestion.
Removed versionadded/changed annotations for 1.6.
Fixed typo in error message in django.contrib.sitemaps.
Fixed #22293 -- Avoided renaming many-to-many tables to themselves.

Fixed this for both implementations of _alter_many_to_many, instead of
in `alter_db_table` itself (more implementations).
Fixed #22257 -- Added file output option to dumpdata command.
Merge pull request #2465 from coder9042/ticket_22258

Fixed #22257 -- Added file output option to dumpdata command.
Fixed #22300 -- Fixed crash in migrations when changing non-relational field to relational.
Fixed #22087 -- Made AdminReadonlyField respect ModelForm Meta overrides.
flake8 and doc fixes for refs #22257.
Fixed #22315 -- str/bytes mismatch in staticfiles

Previously, `ManifestFilesMixin.read_manifest` failed in Python 3
because `json.loads` accepts `str` not `bytes`.