contrib.sites is included in INSTALLED_APPS by SitemapTestsBase.

SessionBase.decode() is the inverse operation to SessionBase.encode().
As SessionBase.encode() always returns a string, SessionBase.decode()
should always be passed a string argument. Fixed the file backend, which
was the only backend still passing a bytestring.

The mysqlclient cursor attribute `_last_executed` is always stored as
bytes. Decode it.

TextField values are already type str. No need to decode.

Support for bytestring templates was removed in
3a148f95.

The old implementation considered objects initialized with an equivalent
signature different if some arguments were provided positionally instead of
as keyword arguments.

Refs #11964, #26167.

Thanks infinite-l00p for the initial patch.

Also renamed the `constraint` argument to `check` to better represent which
part of the constraint the provided `Q` object represents.

Refs #12892 is probably obsolete.

Thanks Rémy Hubscher for the report and Tim Graham and Simon Charette for the reviews.

Thanks Claude Paroz & Tim Graham for collaborating on the patch.

Forward port of d5ed0826 from master.

Upcoming German translations will not differ for past and future naturaltime
translations. Using Czech language instead.