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.

Empieza rastreando cómo mypy gestiona las asignaciones a self en setUp y las declaraciones de variables de clase; después, inspecciona las pruebas que cubren ClassVar y los hooks de configuración de unittest. Define los diagnósticos esperados para cls.foo y self.foo en setUpClass o setUpTestData, y añade pruebas de regresión específicas que demuestren que las asignaciones compatibles ya no producen errores de atributo.