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.

[xssbx]

I suggest a phased approach for Python 3.9 migration and mypy error fixes:

1. Retain Python 3.8+ compatibility first: Keep using  typing.List  instead of native  list[type]  for now, since CI doesn’t support 3.9 yet due to dependency issues. This fixes the mypy errors and keeps the codebase stable.
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2. Prepare for 3.9 migration: Document all places using  typing.List  and create a task to switch to native type hints once dependencies are updated.
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3. Handle  future import annotations : This import is useful for Python 3.7+ type hinting, so keep it—resolve mypy’s dissatisfaction by adding the relevant ignore comments or updating the mypy config file.