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.

Commencez par suivre la manière dont mypy gère les affectations à self dans setUp et les déclarations de variables de classe, puis examinez les tests couvrant ClassVar et les hooks de configuration de unittest. Définissez les diagnostics attendus pour cls.foo et self.foo dans setUpClass ou setUpTestData, et ajoutez des tests de régression ciblés montrant que les affectations prises en charge ne produisent plus d’erreurs d’attribut.