On it's own, this breaks on protocols >= 2 (including Python 3 by
default) because the pickled object's class is required to match the
reduce output. One fix would be to change the pickle format to use a
custom constructor function, but then we'd have different pickle formats
in C and Python.

Rather than do that, we instead create a __class__ property for the
python implementation. This bypasses the protocol limitations. The
__class__ property wasn't assignable in the C implementation, so the
compatibility risk should be very small.

All the special pickle machinery is only defined in Python if needed.