Fixed #16358 - Made memcache backend delete old value on a failure to set.

Default Memcached configuration allows for a maximum object of 1MB and
will fail to set the key if it is too large. The key will be deleted from
memcached if it fails to be set. This is needed to avoid an issue with
cache_db session backend using the old value stored in memcached, instead
of the newer value stored in the database.
Fixed #23806 -- Added a test case for settings.DISALLOWED_USER_AGENTS.
Fixed flake8 warning.
Removed thread customizations of six which are now built-in.
Fixed #23761 -- Added test for MySQL validator and db_type is None.

The issue was fixed on master in e9103402c0fa873aea58a6a11dba510cd308cb84
so this just forwardports the test and release notes from stable/1.7.x.

Forwardport of 2d12a59938 from stable/1.7.x
Updated Fink doc section to be version-agnostic
Removed doc reference to pre-1.5 PostGIS
Added missing docs for 1_7.W001 check.
Fixed #23802 -- Typos in 1.7 release notes.
Fixed #23765 -- Removed BooleanField default check which often yielded false positives.
Fixed #23588 -- Added a link in tutorial for list_display options.
Merge pull request #3481 from manfre/ticket-16358

Fixed #16358 - Made memcache backend delete old value on a failure to set.
Fixed #22407 -- Added AdminEmailHandler.send_mail().
Fixed #23665 -- Noted precedence of settings.USE_L10N in MONTH_DAY_FORMAT and YEAR_MONTH.
Fixed #23827 -- TZ behavior in Storage API ref doc

This is derived from the current behaviour of FileSystemStorage.
Note that since this was not previously documented, other
implementations may not currently conform.
Fixed #23690 - fixed examples of manual rendering of form fields

Documentation for rendering form fields manually is now updated to use fields id_for_label instead of hardcoded values with additional mention of label_tag for alternative generation of complete label tag.
Fixed #23585 - Corrected internal comment.

Removed misleading comment and provide correct one, explaining
idea behind hardcoded CSRF template context processor.
Use topological sort for migration operation dependency resolution

rather than an ad-hoc algorithm
Merge pull request #3525 from vanschelven/master

Use topological sort for migration operation dependency resolution
Merge pull request #3516 from jaylett/filestorage-timezones

Fixed #23827 -- TZ behavior in Storage API ref doc