Fully type module then mark as `py.typed`

[Avasam]

This is an issue to track jaraco.develop being marked as py.typed.
I'd rather not lie about the typed state of a library and wait until all its public members are fully annotated (or at least to a "best effort") before being marked as py.typed.

Refs:

• Expand type hints across projects skeleton#98
• Update mypy.ini from setuptools skeleton#136 (comment)
• Add type hints to setuptools pypa/setuptools#2345