text: Title: Introduce type annotations
  Body: Following @mpenkov's effort for smart_open in https://github.com/RaRe-Technologies/smart_open/pull/545, we could also introduce type annotations into Gensim.
  
  The benefits are mild (nothing much gained), so this would be mostly for the documentation and CI.
  
  Ticket only relevant once we drop py3.6, because without [PEP 585](https://www.python.org/dev/peps/pep-0585/) the annotations are too ugly and code-obfuscating, and PEP585 is py3.7+ only.