Use appveyor to build windows wheels

Use the appveyor continuous integration service to build wheels for
windows. This procedure was adapted from this guide:

https://packaging.python.org/en/latest/appveyor.html

prevent generator methods from returning NULL without setting an exception: method call protocol differs from __next__() protocol

mofe GIVEREF() before PyTuple_SET_ITEM() call because that's when we still own the reference (pypy needs it that way)

fix stupid C-API misuse: accidentally passed a NULL pointer to PyObject_GetItem() instead of a 0 integer index to PySequence_GetItem()