Compatibility with PyPy

Although I would prefer `array.ctypes.data` instead of the `__array_interface__` stuff, I think we should use latter for PyPy compatibility.

Currently, `ndarray.ctypes` is not implemented for PyPy: http://buildbot.pypy.org/numpy-status/latest.html

The `__array_interface__` seems to be deprecated (see https://docs.scipy.org/doc/numpy-1.4.x/reference/arrays.interface.html) but I guess it will stick around long enough for `ndarray.ctypes` to be implemented in PyPy and then we can switch back.

The third commit of this PR is not really necessary for PyPy, but it makes it easy to call `py.test` with PyPy:

    pypy setup.py test