PEP-484 Type Hints

**Is your feature request related to a problem? Please describe.**

We use python with strict typing at our organization which means we have to add a bunch of "type: ignore" comments wherever this library is used. Adding types would make working with this library much easier and require much less context switching between the docs and the editor thanks to autocomplete.

**Describe the solution you'd like**

I understand a bunch of functionality in this library is very hard to type (dynamic clients, etc.), so maybe this effort could start with the JOSE module.

**Describe alternatives you've considered**

Alternatively, type stubs for this library could be added to the [typeshed](https://github.com/python/typeshed) project but that is not ideal as it would be very easy for those to get out of sync.