Add Support for .pyi files

[jackswiney]

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

I have a PyBind11 package that essentially includes *.so c++ source, and *.pyi files for intellisense. It looks like the *.py file extension is hard-coded between this and docspec-python, so the only way to get around this was to change the source to look for the *.pyi extension, or to manually rename my *.pyi files with the *.py extension.

Describe the solution you'd like

I'd prefer a CLI flag to change the file extension, or accept multiple file extensions.

Describe alternatives you've considered

1. Temporarily change the files to the *.py extension, running this tool, then reverting the extensions to *.pyi.
2. Updating the source directly to look for *.pyi OR *.py.
3. Inheriting from the classes in this API and the docspec-python API, then writing custom functions to handle *.pyi.

[PJ-Schulz]

Hello,
I have a use case where I have a large file in my library and adding the docstings to it will increase it more which causes to scroll a lot. Therfore I thought to export the docstrings to .pyi files.
So I have a file called client.py and client.pyi. Maybe on this case the .pyi file should be prefered before the .py file to search for docstings.