Django-timezone-utils

Support Django 4.x

Fixed PyPi distribution to support Python 2 *__and__* 3. No code changes aside from version bump.

For testing push on deploy to pypi.

- Add support for Django 2.1
- Add support for [Python 3.7](https://docs.djangoproject.com/en/2.1/faq/install/#what-python-version-can-i-use-with-django)
- Drop support for [Django 1.8](https://www.djangoproject.com/download/#supported-versions)
- Use tox to run tests and env matrix.

- Removed reference to django.db.models.fields.subclassing.SubfieldBase (resolves 7) and conditional import for `django.core.checks`.
- Removed the `timezone_utils/models.py` and `timezone_utils/migrations`. This means that only Django 1.8+ is now supported.
- Removed support for Python versions < 2.6. The Django 1.6 series was the last to support Python 2.6.
- Added testing support for Django 1.10.
- Coverage fix: calling the super(DateTimeField, self).to_python(value) will convert a naive datetime, so the is_naive condition would never be reached.
- Changed development status from `Beta` to `Production/Stable`.
- Added contributor Alex Kamedov.
- Updated documentation copyright year to 2016.
- Minor PEP8 changes.

Originally released on 2016-05-16, but forgot to tag.