Adding the `--strict` option to mypy

Overview

This is the second part of the discussion from #10065 (review).

Description

I have been working on converting the type hint commentaries into proper annotations (#10004, #10018, #10065, #10074). But I had a simple mistake on #10065 by annotating this:

        download_dir: str = None

when it must be annotated like this:

        download_dir: Optional[str] = None

We expected that Mypy was going to fail, but it just passed by. Then, we saw that Mypy accepts implicit optional annotations, until we use --no-implicit-optional.

What we want

We want Mypy to use --no-implicit-optional on its checks, to make a better diagnosis. View the PR review where the original discussion is for more information.

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[uranusjr]

Assigned

[pradyunsg]

Yea, we have to enable a bunch of mypy’s strictness flags (this is probably the highest impact among those).

The ideal goal would be to get to a point where we can run mypy --strict . and have everything pass.