Add Python typing capability to the library.

[AbstractUmbra]

Hello!

Thank you for creating this library, it is great and easy to use.

I have created this PR which achieves the following, in no particular order:-

• Adds license header to relevant files.
• Clean up around the code base, like __all__ specification and class __slots__.
• Renames some constants to align with standard Python practices.
• Add complete types to the code.
• Add a GitHub workflow that runs on PR or on commit to master to check the types and functionality of the codebase from Python 3.7 -> 3.11.

I didn't want to edit functionality of the codebase for now, but in a future PR I may add a more object-style usage to the return types of the client methods, or at the very least ensure the payload types are known.