Added type to __get__ call to prevent some c modules from segfaulting.

[bakkerthehacker]

Fixes #223

[bakkerthehacker]

Passing in an additional type is documented as the way that descriptors are normally invoked. The second type argument is always present when calling the property from super.

From the 2.7 docs on descriptors:

The call super(B, obj).m() searches obj.__class__.__mro__ for the base class A immediately following B and then returns A.__dict__['m'].__get__(obj, B).

This is really the only issue blocking me from using the backported super throughout my codebase.

[edschofield]

Thanks for your patch, Grant!