Add async support

[Archmonger]

Description

After a bit of thinking, I've determined a way to implement async support in the most minimal way possible.

By creating an AsyncRequestHandler and having variants of the *arrs (ex. AsyncSonarrAPI) use this subclass, this allows for the existing code to return async.

As a result, sonarr/radarr/readarr/etc models will not need to be modified to maintain async support.

Task List

• [x] Create async request handler
• [ ] Create async *arrs that use this new request handler
• [ ] Develop some way of automatically rewriting the type hints for async *arrs endpoints to indicate that they are awaitable. Maybe using a mixture of typing.cast and inspect.getfullargspec?
• [ ] Add tests for async functions

Related issues

• fix #149

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] CI / Tests update

How has this been tested

• [ ] I have added new tests where required
• [ ] I have run nox -s tests locally and passed
• [ ] I have tested this feature in a python script