Type annotations

[mamiksik]

I suggest that all future code should be type annotated and I also suggest adding MyPy to CI. This will make the code easier to read for users as well as maintainers, prevent unnecessary errors and generally make code hinting much more accurate.

References:
PEP 484 -- Type Hints => https://www.python.org/dev/peps/pep-0484/
MyPy => https://github.com/python/mypy

[phanak-sap]

Note: Current list of type checkers in Python to choose from is: Mypy, Pyre, Pyright.

As (perhaps practical, not exact) alternative to marking the code with type annotations, one can introduce also mutation testing on top of current unit tests.