Fix mypy errors in the entire codebase

[dhruvmanila]

We need to fix all the mypy errors and they are increasing with the number of commits. I am planning to open an issue indicating this problem and asking for help. Some questions in my mind:

• As Python 3.9 is out, do we switch to list[type] or keep typing.List? Note: We do not yet run Python 3.9 on our CI as some of the dependencies aren't compatible yet.
• Some files do contain from __future__ import annotation and using list[type] but mypy complains about it:

backtracking/minimax.py:15: error: "list" is not subscriptable, use "typing.List" instead

So, do we switch to Python 3.9 compatible type hints or advise users to user the typing module instead?

[cclauss]

At this point, we should switch to Python 3.9 (dropping any dependencies which are not ready) and then mypy should accept the syntax without the future import.

[dhruvmanila]

That would probably fail some of the checks as some files could rely on that dependency.