Add type hints to the Python code

[mhucka]

There are no type hints in the code, and there should be.

[panta-123]

Option:

1. mypy: https://mypy.readthedocs.io/en/stable/
2. pylyzer: mtshiba.github.io/pylyzer/
   mat be others...

mypy seems to be very popular. Let us know in comments which one do we choose.

[mhucka]

Mypy seems more well-known and also seems good. It was going to be the one I would have looked at first, if I had time to work on this.