Suggest making protocol attribute read-only if invariance causes conflict

If a concrete attribute has a narrower type than protocol attribute, mypy will complain (adapted from #5998):

prog.py:59: error: Argument 1 to "f" has incompatible type "C"; expected "Proto"
prog.py:59: note: Following member(s) of "C" have conflicts:
prog.py:59: note:     name: expected "Optional[str]", got "str"

It could help if mypy would suggest making the protocol attribute read-only (property or final attribute), which is often the right thing to do.

We already generate a related suggestion to use a covariant collection type when using an invariant collection causes type incompatibility.

Beginne damit, den bestehenden Hinweis zur Verwendung eines kovarianten Collection-Typs nachzuverfolgen, wenn invariante Collections eine Inkompatibilität verursachen. Füge einen entsprechenden Hinweis hinzu, das Attribut eines kollidierenden Protokolls schreibgeschützt zu machen, und verifiziere die Diagnose mit einer Regression-Abdeckung für das gezeigte Beispiel.