Reports invalid usages of a class with __slots__ definitions.

Example when accessing undefined attributes:

class Foo:
    __slots__ = ['foo', 'bar']

    def __init__(self):
        self.x = 3  # error: 'x' is not defined in __slots__

Example of conflicting attributes:

class A:
    __slots__ = ("x",)
    x = 42  # error: conflict with "x" listed in __slots__

Example slots=True:

from dataclasses import dataclass

@dataclass(slots=True)  # error: __slots__ is also defined in Foo
class Foo:
    __slots__ = ['a']