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.