Fixed some flake8 issues
Fixed #21491 -- Removed documented workaround for a known issue.

The issue was that two M2M hidden reverse managers
(related_name ending with a '+') could clash with each other.

Refs #21375 and #15932. Thanks Baptiste.
Merge pull request #1965 from loic/ticket21491

Fixed #21491 -- Removed documented workaround for a known issue.
Fixed #21443 -- Cannot show debug info on PY3's importlib

Thanks productions@zaziork.co.uk for the review.
Removed unused import in docs/topics/http/sessions.txt
Noted that localmem is the default cache.
Improved docs for ModelFormSet.clean().
Fixed #21012 -- Thread-local caches, like databases.