Improving gensim type hints?

[J.I.]

Hi there,

I started using gensim this weekend and noticed type hints and IDE-autocomplete features were lacking. 

I like using Visual Studio Code with the setting `python.analysis.typeCheckingMode` set to `strict` and right now the gensim package produces dozens of warnings and errors under this setting because the package has few type hints and no .pyi stub files. 

Would the maintainers welcome contributions that add better type hints to gensim? It'd be very nice to see improved type hint functionality in the next release and I'd be very happy to work on that over 1 or 2 PRs this week.

Do let me know if that kind of work would be welcome!

~J.I.