COMPAT: avoid getsizeof on PyPy
Pandas uses ``sys.getsizeof()`` to implement ``memory_usage()``. PyPy does not implement this functionality by design, since memory size is not fixed, depending on the JIT and other optimizations the memory usage of an object can change.

I would like to suggest one possible solution: using the default in sys.getsizeof(obj, default). If this is unacceptable, I could suggest that using ``deep=True`` would raise a ``TypeErrpr`` instead, but that would be more complicated. Any opinions? (proof-of-concept pull request forthcoming)