Update error message for incompatible args

[matys18]

Closes #5025 🛠

Updated the error message that is displayed when the argument types of a method in a subclass are incompatible with the arguments of the same method in the superclass.

Since the output message has changed there are quite a few broken tests. I would like to review and agree on the wording before changing them.

# sample.py

class A:

    def kek(self, lol: int = 0) -> None:
        print("Kek in A")


class B(A):

    def kek(self, haha: str = "") -> None:
        super().kek(1)
        print("Kek in B")

Output before:

mypy sample.py
sample.py:11: error: Argument 1 of "kek" incompatible with supertype "A"

Output now:

mypy sample.py
sample.py:11: error: Argument 1 of "kek" is incompatible with supertype "A"; supertype defines the argument type as "int"

As pointed out by @ilevkivskyi incompatible return type error reporting could also benefit from the same treatment. Will enter a followup issue once this is merged.

[ilevkivskyi]

The general idea looks good. It looks like you forgot to add/update tests however. Also there is one comment below.

[ilevkivskyi]

Thanks for updates! LGTM now.