[1.7.x] Fixed an instance of 'an' before a word beginning with a consonant.

Backport of ef8ef2a42d from master
[1.7.x] Fixed #23452 -- Prevented infinite migrations for empty unique/index_together.

Thanks fwkroon for the report.

Backport of 6d5958c7a3 from master
[1.7.x] Corrected grammar in migrations error message.

Backport of 61f56e239f from master
[1.7.x] Fixed typo in topics/forms/formsets.txt

Backport of e265112cd1 from master
[1.7.x] Corrected indentation in a contrib.humanize test; refs #23340.

Thanks Zach Borboa for the report.

Backport of 96010ae15b from master
[1.7.x] Avoided using deprecated version of importlib; refs #23418.

Backport of fbeb7979ee from master
[1.7.x] Added some items to the 1.7.1 release notes.

Backport of cc5e81c46d from master