Support adding class variables to class through assignment

Mypy supports adding items to self during test setUp, which is great. However in many cases it's useful to be able to add items to the class during setUpClass (or Django's setUpTestData).

import unittest

class Tests(unittest.TestCase):
    @classmethod
    def setUpClass(cls) -> None:
        cls.foo = 42  # "Type[Tests]" has no attribute "foo"

    def test_foo(self) -> None:
        print(self.foo)  # "Tests" has no attribute "foo"

Adding a ClassVar allows for a workaround to this, however it would be nice if that wasn't needed.

Beginne damit nachzuverfolgen, wie mypy Zuweisungen an self in setUp und Deklarationen von Klassenvariablen behandelt, und untersuche dann Tests, die ClassVar und unittest-Setup-Hooks abdecken. Lege die erwarteten Diagnosen für cls.foo und self.foo in setUpClass oder setUpTestData fest, und füge fokussierte Regressionstests hinzu, die zeigen, dass die unterstützten Zuweisungen keine Attributfehler mehr erzeugen.