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 #12030 -- Validate integer field range at the model level.

Thanks to @timgraham for the review.