Support slots=True in attrs

[sobolevn]

Similar to #11483 we need to support this case:

import attr

@attr.dataclass(slots=True)
class A:
   x: int

   def __init__(self, x :int) -> None:
       self.x = x  # ok
       self.y = 0  # slots error

Source: https://github.com/python/mypy/blob/master/mypy/plugins/attrs.py#L272-L277

I am going to send a PR today.