Fixed #23957 -- Started deprecation toward requiring session verification.

Thanks Carl Meyer for review.
Fixed #23955 -- Corrected formset fields in topic guide.
Corrected example in Form.has_changed() docs.
Refs #23947 -- Made a modeladmin test cleanup after itself.
Fixed documentation of GeoModelAdmin.openlayers_url
Fixed typo in docs/ref/applications.txt.
Cleaned up a note in docs/topics/db/sql.txt.
Fixed #23958 -- Rewrote filter tests as unit tests.
Refs #23890 -- Restored silencing of numpy DeprecationWarnings in template tests.
Fixed #23965 -- Mentioned that FieldFile inherits from File
Refs #23947 -- Isolated a mail test.
Refs #23947 -- Fixed introspection test that had a side effect.
Refs #23947 -- Isolated some cache tests.

Thanks Diego Guimarãesi and Florian Apolloner.
Updated obsolete comment.
Moved misplaced comment.
Refs #23947 -- Worked around a bug in Python that prevents deprecation warnings from appearing in tests.
Add missing dot
Merge pull request #3702 from rutsky/patch-1

Add missing dot
Fixed typo in docs/howto/custom-lookups.txt.
Added an "htmlview" target to open docs after building them.

This is helpful if you build and test documentation updates in
your development env.

Copied from CPython's Doc/Makefile.