Not all types and nodes have `__slots__`

[sobolevn]

For some reason not all classes in

• types.py https://github.com/python/mypy/blob/master/mypy/types.py
• nodes.py https://github.com/python/mypy/blob/master/mypy/nodes.py

have explicit __slots__.

I will send a PR with the fix.