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.