Adding type hints to channel_redis

[LuisGMM]

Hey folks, great work here, thanks a lot!

I was wondering if there are any plans to add type hints to the project :D

[bigfootjon]

We're open to it, but no one is actively working on it.

If you're interested in working on this, I'd be delighted to review Pull Requests.

The basic plan I have is:

1. Enable mypy inside tox (like how it works on asgiref: https://github.com/django/asgiref)
2. Add types to a few simple (obvious) cases
3. Progressively add more and more coverage
4. Add a py.typed file once we get enough coverage

Each step should be a separate PR (3 can be many PRs), so if you just want to get the ball rolling with 1 and maybe 2 I think that would be great!

[iwakitakuma33]

@bigfootjon
This is my first time participating in this repository.
I apologize in advance if I have violated any rules.
I have implemented typehints, so could you please review it?

#417