MySQL does allow primary key with value 0. It only forbids autoincrement
primary key with value 0.

Thanks Claude Paroz for the report.

This is the result of Christopher Medrela's 2013 Summer of Code project.

Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.

Also: Fixes #8579, fixes #3055, fixes #19844.

Thanks agale031176@gmail.com for the report.

Signed-off-by: Jason Myers <jason@jasonamyers.com>

Should be unneeded with Python 2.7 and up.
Added some unicode_literals along the way.

This also updates all dependent functionality, including modelform_factory
 and modelformset_factory, and the generic views `ModelFormMixin`,
 `CreateView` and `UpdateView` which gain a new `fields` attribute.