[1.7.x] Fixed flake8 warning.

Backport of dec7da3c4c from master
[1.7.x] Fixed #23761 -- Fixed crash with MySQL validator and db_type is None.

The issue was fixed on master in e9103402c0fa873aea58a6a11dba510cd308cb84.
[1.7.x] Removed thread customizations of six which are now built-in.

Backport of 7ef81b5cdd from master
[1.7.x] Updated Fink doc section to be version-agnostic

Backport of 7fd1f3a645f from master.
[1.7.x] Added missing docs for 1_7.W001 check.

Backport of 098368d82f67715723b5bd4445a8fe92a0ceae98 from master
[1.7.x] Fixed #23802 -- Typos in 1.7 release notes.

Backport of d5a109f6e6684b2ea8028af5c57f4257872d11aa from master
[1.7.x] Fixed #23588 -- Added a link in tutorial for list_display options.

Backport of 3a550bb6d32172fc1ec5dd314d20ef1235db995a from master