Fixed "redefinition of unused 'foo' from line X" pyflakes warnings.
Fixed an undefined variable and clarified the purpose of a test.

refs #4459.
Whitespace cleanup.

* Removed trailing whitespace.
* Added newline to EOF if missing.
* Removed blank lines at EOF.
* Removed some stray tabs.
Sanitized test assertions in gdal tests
Small improvements to docs/topics/db/models.txt
Fixed assorted flake8 errors.
Fixed #8918 -- Made FileField.upload_to optional.

Thanks leahculver for the suggestion and dc and vajrasky for work
on the patch.
Fixed #13252 -- Added ability to serialize with natural primary keys.

Added ``--natural-foreign`` and ``--natural-primary`` options and
deprecated the ``--natural`` option to the ``dumpdata`` management
command.

Added ``use_natural_foreign_keys`` and ``use_natural_primary_keys``
arguments and deprecated the ``use_natural_keys`` argument to
``django.core.serializers.Serializer.serialize()``.

Thanks SmileyChris for the suggestion.
Added tests for URLValidator schemes.
Minor edits to docs/topics/db/queries.txt.
Fixed errors in 848101bf1736d1c01eb86968e85a14c7466bb376
Fixed #15582 -- Documented how TransactionTestCase.multi_db affects fixtures.

Thanks slinkp for the suggestion.
Updated OpenLayers version from 2.11 to 2.13
Forms docs: More emphasis in 'id'  value customizability.
Fixed #21173 -- Stopped fixing format for date-based widgets at init time

Thanks Marc Tamlyn for the review.
Fixed #19560 -- Identified field in warning for naive datetime.

Thanks gcc for the report and vajrasky for the patch.
Fixed #21198 -- Prevented invalid use of @python_2_unicode_compatible.

Thanks jpic for the report and chmodas for working on a patch.

Reverts 2ea80b94. Refs #19362.
Fixed #21256 -- Error in datetime_safe.datetime.combine.
Imported custom user classes in tests depending on it

Without those imports, affected test files cannot be run
independently. Refs #21164.
Amend my description in committers.txt.