[skip ci] .gitignore: exclude __pycache__ directories (#5838)

Python may leave behind temporary `.pyc.*` files inside `__pycache__`
on some filesystems (e.g. WSL2 mounts). Adding `__pycache__/` ensures
these directories and any leftover files are consistently ignored.

Background: Python writes bytecode to a temp file with an extra suffix
before renaming it to `.pyc`. If the process is interrupted or the
filesystem rename isn’t fully atomic, those temp files may remain.

See: https://docs.python.org/3/library/py_compile.html#py_compile.compile
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