Fixed #22680 --  I/O operation on closed file.

This patch is two-fold; first it ensure that Django does close everything in
request.FILES at the end of the request and secondly the storage system should
no longer close any files during save, it's up to the caller to handle that --
or let Django close the files at the end of the request.
Removed an unused local variable, fixes a flake8 error
Fixed #21430 -- Added a RuntimeWarning when unpickling Models and QuerySets from a different Django version.

Thanks FunkyBob for the suggestion, prasoon2211 for the initial patch,
and akaariai, loic, and charettes for helping in shaping the patch.
Fixed #22068 -- Made runtests.py remove trailing slashes from test labels.

When using tab-completion it's easy to accidentally run a test with
a trailing slash, which causes INSTALLED_APPS to be set incorrectly.
Normalize the test labels to avoid this common error.

Thanks MattBlack for the suggestion.
Created a new tests folder (`model_options`).

And moved `tablespaces` option tests to it.
The new folder can be used to test models/options, like the new option
added in refs #22778.
Fixed #22653 -- Added some database feature flags to tests.

Thanks Rahul Priyadarshi.
Fixed #22487: Separated schema migration from data migration in test

The data migration failed on Oracle, killing the entire test suite.

Thanks timo for reporting the Oracle breakage,
and andrewgodwin for suggesting the solution.
Added link to data migrations in initial data deprecation note
Added note to alphabetize AUTHORS.
Fixed typo in docs/ref/forms/validation.txt.