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!
We probably don't need to have the Python wrapper objects in their own .h file, since they aren't used anywhere else.  `_backend_agg_wrapper.h` exists because the `_tkagg.h` backend has to pull things out of the Python objects in a non-Python thread, but that's a very exceptional case, and most Python wrappers should not have header files -- that encourages a tight-coupling of the modules which the PyCXX-removal aims to do away with.