bpo-33370: Add .mypy_cache to .gitignore

[cypher-naman]

Added .mypy_cache to .gitignore

Mentored by @CuriousLearner

https://bugs.python.org/issue33370

[the-knights-who-say-ni]

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[matrixise]

I am sorry, but I don't see why we need to have .mypy_cache in the .gitignore file...

As @brettcannon has said, you have to modify your global .gitignore and not the .gitignore of this project.

Have a nice day,

[ethanhs]

There is already an open PR here that passes CI #6614. I also don't understand why this is needed?

[brettcannon]

Closing this as a duplicate of #6614.