No compatible with `__slots__`

Hi. Very interresting project. I was about to develop the same thing, but made a research first and found this

The problem is that it doesn't support the use of __slots__

Demonstration:

>>> from cached_property import cached_property
>>> class C:
...     __slots__ = 'x',
...     @cached_property
...     def y(self):
...         return 'y'
... 
>>> c = C()
>>> c.y
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/site-packages/cached_property.py", line 26, in __get__
    value = obj.__dict__[self.func.__name__] = self.func(obj)
AttributeError: 'C' object has no attribute '__dict__'

I have a little wrapper for this named slot_wrapper somewhere.

Here is a simplified version:

def slot_wrapper(attrname, slot, cls):

    class InnerDescriptor:

        def __get__(self, obj, cls):
            return slot.__get__(obj, cls)

        def __set__(self, obj, val):
            slot.__set__(obj, val)

return InnerDescriptor(attrname)

It takes the descriptor under the slot and replace it (__slots__ underly special descriptors, but they remains descriptors after all). It needs adjustments with your code in order to work... I use it with metclass so it got instaciate after the creation of a class wich have __slots__.

My demand is between a bug report, a feature request and an enhacement proposal!