Generate/add stubs for PEP561 and add py.typed

[segevfiner]

Need to figure out a way to extract the types from the Cython module to a *.pyi stub file and add a py.typed file. This should enable autocompletion and type checking.

This will require converting from a single file module to a package though. Not sure whether to place all code in an __init__.pyx or include an internal _cypcap.pyx and have __init__.py do a from ._cypcap import *

[segevfiner]

This also depends on either finding a third party way to do this or for Cython to add builtin support.

[segevfiner]

Apparently neither mypy nor pyright can successfully generate a draft stub file from a native Cython module 🤷‍♂️

[segevfiner]

It seems there is some bizarre conflict here where pylance/pyright expects typing files to include docstrings and default values, but none of the existing ones do...

[FeldrinH]

There seems to be some relevant progress in cython/cython#3818 and cython/cython#5744. I would love to see this issue resolved, because as it is right now, my IDE fails to find any type info for cypcap and considers every reference to a cypcap method or class an error.

[FeldrinH]

In the meantime it might also be worth checking out the existing .pyx -> .pyi converters mentioned here: cython/cython#3818 (comment)