Unicode method on model should return unicode

The example of a __unicode__ model method should return a unicode value
instead of a string.

Thanks Daniel Pope for the suggestion.

Thanks Rudolph for the report.

Removed the FieldError raised by ModelBase.__new__ in this case.

This reverts commit 9d1987d7.

Thanks Tim Graham for the report and Simon Charette for the review.

refs #19195.

Thanks to Trac alias tbartelmess for the report and the test project.

Fixed #20550 -- Added ability to preserve test db between runs

This reverts commit cb9c9a7b.

Note that the stdlib's version of pbkdf2_hmac will also be used in the upcoming Python 2.7.8 release

Thanks apollo13 for the review.

Thanks matklad for the report.

Fixed #22682 -- `makemigrations` will create `MIGRATION_MODULES` package

`makemigrations` will automatically create the package specified
in `MIGRATION_MODULES` if it doesn't already exist.

Thanks ovidiuc4 for the report.

Making SQL management commands migration aware.

Thanks Simon Charette and Tim Graham for the reviews.

Fixed #22720 -- Migrations attempt to create _order twice.

Python 2.7.7 includes compare_digest in the hmac module, but it requires
both arguments to have the same type. This is usually not a problem on
Python 3 since everything is text, but we have mixed unicode and str on
Python 2 -- hence make sure everything is bytes before feeding it into
compare_digest.