Fix false-positive assigning-non-slot with typing.Generic base

[cdce8p]

Description

Fix a false-positive for assigning-non-slot with base that inherits from typing.Generic.

from typing import Generic, TypeVar
T = TypeVar("T")

class Base(Generic[T]):
    __slots__ = ()

class Foo(Base[T]):
    __slots__ = ['_value']

    def __init__(self, value: T):
        self._value = value

The reason for the issue is that slots() is evaluated and cached before any inference tips from astroid are applied.
For classes that directly inherit from Generic this is fixed by astroid, everything else needs to be handled by pylint at least for now. -> astroid fix

Type of Changes

	Type
✓	🐛 Bug fix

Related Issue

Closes #4509
Closes pylint-dev/astroid#999

[cdce8p]

/CC: @hippo91

[coveralls]

Coverage increased (+0.002%) to 91.835% when pulling 721d9c3 on cdce8p:fix-assigning-non-slot into d09c5de on PyCQA:master.

[Pierre-Sassoulas]

👍