.pyi type hints

[amunra]

We have a Cython package that doesn't do a great job of exposing typing information to PyCharm.
It looks like this needs additional .pyi files to get it working:

https://stackoverflow.com/questions/41734836/what-does-i-represent-in-python-pyi-extension

Shame, since pretty much all the typing is already on the signatures in Cython and usually works just fine in VS Code. Oh well :-)

[amunra]

This is a low priority for us, since it doesn't really impede anything from working, but it's an area that we'd welcome a contribution.

[nunnsy]

Have had an initial crack at this here: #95