Patches for matplotlib Python 3.x support

[Michael D. <md...@st...>]

I'm not sure what the procedure for submitting patches to PyCXX is, but 
I wanted to bring to your attention some patches that are required to 
successfully build matplotlib on Python 3.x.  We currently ship our own 
copy of PyCXX with these patches, but we'd like to stop doing so in the 
future, of course.

This is to support the buffer interface:

https://sourceforge.net/tracker/?func=detail&aid=3590190&group_id=3180&atid=303180

This is to use Py_hash_t rather than long where appropriate, because 
sizeof(long) != sizeof(long *) on some platforms:

https://sourceforge.net/tracker/?func=detail&aid=3590194&group_id=3180&atid=303180

And lastly, and least importantly, a simple patch to suppress a compiler 
warning about compare_handler being defined but not used:

https://sourceforge.net/tracker/?func=detail&aid=3590198&group_id=3180&atid=303180

Cheers,
Mike