Prevented queries from reusing trimmed joins.

Thanks Luke Sneeringer for the suggestion.

The test from refs #20368 only runs this block on Python 2.

Thanks Christian Ullrich for the report and review.

Thanks Mariusz Felisiak and Tim Graham for polishing the patch.

Thanks Tim Graham and Nick Pope for review.

Thanks Tim Graham for the review.

Thanks Tim Graham for the review.

Using __str__() and then repr'ing the result looks strange and can lead
to recursive rendering of forms.

Caused by `contrib/auth/backends.py` changes in
cb7bbf97.

Reverted the obsolete fix and tests for refs #12302.

The new test crashed in the removed branch. It's unclear if the branch has
value since c6a2bd9b didn't include tests.