Make type checking mypy more straightforward for new contributors

There are a couple pitfalls relating to type checking mypy itself that don't seem to be documented anywhere:

1. Specifying files: Running mypy . and mypy mypy mypyc both result in duplicate module errors that prevent further type checking. The correct way to run mypy is mypy -p mypy -p mypyc, which doesn't appear to be documented anywhere (I only found this by looking at the tox.ini config file).
2. Specifying the config file: Failing to specify mypy_self_check.ini as the config file results in an error in mypy/bogus_type.py and means that people aren't using mypy's stricter type checking options.

I think the solution to 2 would be to rename mypy_self_check.ini to mypy.ini, because self checking mypy is significantly more common than compiling mypy with mypyc, so making mypy_self_check.ini the default config file seems reasonable.

For 1, we should at the very least document the command to run mypy in CONTRIBUTING.md. I think the ideal solution would involve changing the configuration file so that either mypy . or mypy mypy mypyc just works correctly, the latter of which seems to be possible from my experiments by turning on namespace_packages and excluding mypyc/test-data.

Beginne mit CONTRIBUTING.md und tox.ini, um die dokumentierten und konfigurierten mypy-Befehle zu vergleichen. Untersuche mypy_self_check.ini und mypy/bogus_type.py und prüfe anschließend, ob die vorgeschlagenen Änderungen an der Konfigurationsbenennung oder an namespace packages Fehler durch doppelte Module vermeiden und gleichzeitig die strikte Selbstprüfung beibehalten. Als erledigt gilt die Aufgabe, wenn der Beitragsworkflow einen zuverlässigen Befehl und eine zuverlässige Konfiguration dokumentiert.