Django-channels-graphql-ws

- Python 3.6 compatibility brought back.

- Subscription `payload` now properly serializes the following Python
`datetime` types:
- `datetime.datetime`
- `datetime.date`
- `datetime.time`

- Allow `msgpack v1.*` in the dependencies requirements.
- Windows support improved: tests fixed, example fixed.
- Development instructions updated in the `README.md`.
- Removed `graphql-core` version lock, it is hold by `graphene` anyway.
- Many CI-related fixes.

- Added support for Python 3.6.
- Dependencies updated and relaxed, now Django 3 is allowed.
- Testing framework improved to run on 3.6, 3.7, 3.8 with Tox.
- Client setup documentation (Python, GraphiQL) improved. Thanks to
Rigel Kent.
- Error logging added to simplify debugging, error messages improved.
- Fixed wrong year in this changelog (facepalm).
- Configuration management made slightly simple.
- Bandit linter removed as useless.
- More instructions for the package developers in the `README.md` added.

- Example improved to show how to handle HTTP auth (23).

- Better error message when Channels channel layer is not available.