Django-channels-graphql-ws

- Logging slightly improved. Thanks to edouardtheron.

- Quadratic growth of threads number has stopped. The problem was
observer on Python 3.6 and 3.7 and was not on 3.8, because starting
with 3.8 `ThreadPoolExecutor` does not spawn new thread if there are
idle threads in the pool already. The issue was in the fact that for
each of worker thread we run an event loop which default executor is
the `ThreadPoolExecutor` with default (by Python) number of threads.
All this eventually ended up in hundreds of thread created for each
`GraphqlWsConsumer` subclass.

- 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.