Complete removal of PyCXX

This PR completes the removal of PyCXX from matplotlib following on from PR #3646.

There are 2 commits.  The first converts the _tri module, the last module to use PyCXX, to use the Python/C API instead.  I've followed the lead of #3646 so that we have some code consistency in terms of separate wrapper files, code layout, use of the same exception-catching macros, etc.

The second commit removes the PyCXX source code, the relevant sections of the setup scripts, and a few other places where it was mentioned.

It is always nice to write a PR that is such a net reduction in code!
When I made my first comment here, I didn't quite realise all of the issues here.  I guess in this case, I'm fine was as-is, or what you suggest.  All I was really trying to avoid was other modules using the _tri_wrapper.h header (by not making it available), but that's just trying to avoid a problem we don't have...  Use your judgement here.