Port Python 3 to Git and bump to 3.7.1

Closes #507 (closed)

For ABI break see https://bugs.python.org/issue34762

Unfortunately, the current C API for PEP 567 has a flaw: it uses non-PyObject pointers.  

This causes problems with integrating with tools like Cython, where PyObject is special and a lot of machinery recognizes it and manages refcounts correctly.

It also goes against the recent push to improve C API; one of the things we want to fix is to eliminate non-PyObject pointer types from public APIs entirely.

Because this C API is new (landed in 3.7.0) I think it might make sense to change it in 3.7.1.  I *think* this is a relatively safe (albeit annoying) change
...
If we don't change this API now, we'll likely have to either live with it, or face a very slow deprecation period.