Warn if instance atribute is defined outside of __init__

Feature

(Optionally?) generate error if the instance attribute is defined outside of __init__

Pitch

Mypy does not generate any errors for the code below:

class Tst:
    def __init__(self) -> None:
        ...

    def create_field(self) -> None:
        self.abc = 1  # Please, generate error here

    def print_field(self) -> None:
        print(self.abc)  # And, optionally here...


def main() -> None:
    tst = Tst()
    tst.print_field()


if __name__ == '__main__':
    main()

which fails with run time error, caused by the absence of the abc attribute definition in the __init__ method.

Additionaly:

PyCharm and pylint generate error in the line 6.

The issue names no implementation files, tests, or entry points. Start with the supplied Python example and review how mypy currently handles instance attributes; done means agreeing on a diagnostic for assignments outside init and whether reads such as print_field are included, then covering the requested behavior with tests.