Typed-config

Drop running tests on Python 3.6 since it is end of life.

Enable CI testing on Python 3.10 and 3.11. Use `sys.version_info` to help with imports as mypy plays nicely with this.

Fix `ImportError` for `typing_extensions` on Python <= 3.7, by requiring it as a dependency. This is because `Literal` is used as a type. This did not become part of the built in `typing` module until Python 3.8.

Remove pointless casting of `default` value, since the `default` should be provided already cast, and should have the same type as the return value of the `cast` function. The also means that the type of a custom `cast` function should only need to accept a string as input, rather than having to check for if it has already been cast.

Make type annotations pass string Mypy checking.

Remove `include_package_data=True` from `setup.py` to ensure `py.typed` is included in source distributions