This repository seems to be unmaintained for some years now and python 3.8 broke the initialization code.
Since no one can merge patches into the main repository, I forked this repo and added a few fixes.
A working version of matplotlib-cpp with python >= 3.8 can be found here:

https://github.com/amadeus84/matplotlib-cpp

Not the prettiest code, but we'll clean it up at some point.

Disclaimer: I do write python code and C++ code, but I'm not a core python developer so to call python from C++ I rely on existing code and google. Improvements from people more familiar with the python internals and with the python C API are welcome.

Amadeus