[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

.gitignore

@@ -26,6 +26,7 @@ MANIFEST
 /.ninja_*
 /*.ninja
 /docs/.build
+__pycache__/
 *.py[co]
 *.egg-info
 *~