* Renamed the __unicode__ methods
* Applied the python_2_unicode_compatible decorator
* Removed the StrAndUnicode mix-in that is superseded by
python_2_unicode_compatible
* Kept the __unicode__ methods in classes that specifically
test it under Python 2
Databases with update_can_self_select = False (MySQL for example)
generated non-necessary queries when saving a multitable inherited
model, and when the save resulted in update.
MySQL generates an extra query in inheritance cases when doing an update.
This results in a test failure when checking for number of queries in
update_only_fields tests. Added a skip temporarily to avoid this test
failure. Refs #18304.
Added the ability to update only part of the model's fields in
model.save() by introducing a new kwarg "update_fields". Thanks
to all the numerous reviewers and commenters in the ticket
