Add "is invariant" warning to Set as well as List for situations where a Collection should be used

Feature

mypy has a lovely warning about List being invariant in situations like this:

ls: List[int] = [1, 2, 3]

def process(stuff: List[Union[int, str]]):
    ...

process(ls)  # type error here, w/ nice explanation of invariance and suggestion to use Sequence

However, it does not have the same warning when Set is used in the same way:

s: Set[int] = {1, 2, 3}

def process(stuff: Set[Union[int, str]]):
    ...

process(s)  # merely states that the type is incompatible, no mention of invariance

Would be nice if the same warning mechanism could trigger here, suggesting Collection.

Pitch

Hopefully this is a simple widening of a rule that's special-cased for lists to get sets too? Not sure if other types like Dict/Tuple might be included too; I haven't tested them. If it's a lot of effort probably not worth it.

Beginne damit, die bestehende Warnung zur Invarianz von List und ihre Tests zu finden; das Issue nennt keine bestimmten Dateien oder Einstiegspunkte. Erweitere dasselbe Warnverhalten auf Set und verifiziere, dass die Diagnose die Invarianz erklärt und Collection vorschlägt, wie im Beispiel gezeigt.