[1.7.x] Fixed typo in multipartparser.py

Backport of 8a9d54aa69 from master
[1.7.x] Fixed #22628 -- Took initial forms into account when combining FormSet.min_num and FormSet.extra.
[1.7.x] Fixed #22579 -- Corrected validation for email to reject trailing slash

Backport of 424fe76349a2e34eafef13c2450a7a1f4d3115a6 from master.
Fixed #22557 -- ManifestStaticFilesStorage did not cleanup deleted files

When using ManifestStaticFilesStorage, deleted static files would be
correctly cleaned up by "collectstatic --clear", but the manifest file
would still contain the stale entries.

Thanks to tedtieken for the report