Fixed #23883 -- Stopped flatatt modifying its argument
Corrected Permission.max_length in docs; refs #8162.
Added a test for model pre/post_init signals.
Fixed #23794 -- Fixed migrations crash when removing a field that's part of index/unique_together.
Fixed #23863 -- Made runtests accept the keepdb option.

refs #20550
Fixed typo in docs/internals/release-process.txt.
Fixed #23865 -- documented how to assign errors to a field in Model.clean()

Also added a unit test wit the simpler syntax which we have documented,
where the dictionary values are strings.
Fixed #21753 -- Raised exception when both `form_class` and `fields` are specified.
Fixed #23778 -- Added a doc section on using the Django runner for reusable apps.
Fixed typo in forms/models.py comment.
Fixed #23862 -- Made ManyToManyRel.get_related_field() respect to_field.
Fixed #23889 -- Added more usage of quote_name_unless_alias() to avoid deprecation warnings.