[1.7.x] Fixed minor syntax error in Queryset docs

Backport of f2eea960e052db2d280e6dd016b0f8f23d5a8ef7 from master.
[1.7.x] Fixed #22255 -- Added support for specifying re flags in RegexValidator

Backport of 4d0c5f61427a8e67552ee2d777fffbadc7aff3b2 from master.
[1.7.x] Fixed flake8 warnings.

Backport of 35f46ec7a9 from master
[1.7.x] Replaced an instance of PIL with Pillow in the docs.

Done in 4965a774074780f3e4858bcc975476f71edf2c2c on master.
[1.7.x] Fixed typos in docs (django.contrib.site)

Backport of 907ac646415a911b54d84c5203d4fa78ed1b91d5 from master.
[1.7.x] Small doc changes missed in 66076268.

Partial backport of 253e8ac2 from master.