Add type hints

[KapJI]

• Setup mypy.
• Enable strict checks.
• I used modern PEP 585 type hints, like dict[str, str]. Old ones like Dict will be deprecated in Python 3.9 and removed completely in 2025. In Python 3.8 this requires from __future__ import annotations, this will be enabled by default in Python 3.10. One of the advantages is that these new types don't need to be imported.
• Unfortunately, urllib3 doesn't export typing information yet, this is tracked by Start shipping our own type stubs urllib3/urllib3#1897. I'll try to update gpsoauth when it's ready.
• Include py.typed marker to export all typing information.
• Add Typing :: Typed classifier.
• Explicitly add urllib3 to dependencies list.
• Use standard importlib.metadata from Python 3.8.
• Bump version to 0.5.0.

Closes #26

[simon-weber]

looks good; thanks!