Add type hints

Right now, when using this library in any project which uses mypy, it gives the following error:
```
error: Skipping analyzing "m3u8": module is installed, but missing library stubs or py.typed marker
```

That's because the library doesn't have type hints, nor have type stubs in the [typeshed repository](https://github.com/python/typeshed).

Type hints are super helpful, not only can help to prevent bugs when combined with a static code analyzer like mypy, but also serves as documentation for any new user of the library, which makes it far easier to understand what can be and cannot be done with every method and property.

Please, consider adding type hints to the library.

Start by reviewing the library's public API and existing test setup, then reproduce the mypy error described in the issue. Determine the scope of annotations needed across the library and verify completion by checking that consumers no longer report a missing stubs or py.typed marker error.

Written by the indexing model from the issue text.