Add `__pycache__/` to `.gitignore`

[Avasam]

Summary of changes

Running tox locally leaves a lot of files behind in __pycache__ folders. This PR adds the cache folder to the .gitignore, since this is specific to this project (using tox) and not my personal environment.

Pull Request Checklist

• [ ] Changes have tests
• [ ] News fragment added in newsfragments/. (no user facing changes) (See [documentation][PR docs] for details)

[PR docs]: https://setuptools.pypa.io/en/latest/development/developer-guide.html#making-a-pull-request

[abravalheri]

https://blog.jaraco.com/skeleton/#ignoring-artifacts does mention __pycache__ in the suggested .gitignore_global, so by that logic, this line would not be necessary...

However there is precedent (the currently ignored *.pyc, .mypy_cache, .pytest_cache, etc...). So I will defer this one to Jason 😅.