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.

Unused since its introduction in e7e4b8b0.

Reverted refs #6423 since raising string exceptions is prohibited
since Python 2.5.

Thanks Shai Berger for test help and Adam Johnson for review.

Thanks Simon Charette, Mariusz Felisiak, Tim Graham, and Adam Johnson
for review.

Unneeded since 7b2f2e74.

This will allow to replace force_text() by str() in several places (as one of
the features of force_text is to keep the safe status).