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.

Thanks danielsamuels for the report

While at it, fixed a problem in returning empty values
(still with BinaryField/Oracle/Python3).

Thanks Tim Graham for the report.

Thanks semenov for the report.

`django.get_version` was actually used by the module.

Thanks Aymeric Augustin for the report end the review.

This makes it possible to run django.setup() in management commands that
don't need a settings module. In addition it simplifies error handling.

Thanks Claude for the review.

`makemigrations --dry-run` will output the complete migrations file
that would be written if it's used along with `--verbosity 3`.

Added documentation about `--dry-run` and `--merge`.

Also removed unused import in runtests.py.

Made the fix in InteractiveMigrationQuestioner class code, rather than
MigrationAutodetector, because --dry-run shouldn't affect whether
MigrationAutodetector will detect non-nullable fields, but the
questioner should skip the question and returns a None for default
(since that won't be used anyway) if --dry-run is used.

Refs #8620.

If we allow any value to remove form fields then we get name clashes
with method names, media classes etc. There was a backwards
incompatibility introduced meaning ModelForm subclasses with declared
fields called media or clean would lose those fields.

Field removal is now only permitted by using the sentinel value None.
The docs have been slightly reworded to refer to removal of fields
rather than shadowing.

Thanks to gcbirzan for the report and initial patch, and several of the
core team for opinions.

This also fixes #22686 as some sort of side-effect.

This will also trigger the ready method of any tested application,
which was not run before.

Thanks to rockallite for the report.