When you add new fonts to your system, you need to delete your fontList.cache file in order for matplotlib to find them.
The file fontList.cache is located at your Userfolder –> .matplotlib/fontList.cache or ~/.cache/matplotlib/fontList.cache, for Windows that would normally be C:\Users\yourUsername\.matplotlib\fontList.cache
matplotlib.font_manager matplotlib.font_manager._rebuild()
References
https://stackoverflow.com/questions/26085867/matplotlib-font-not-found
http://andresabino.com/2015/08/18/fonts-and-matplotlib/
https://stackoverflow.com/questions/45877746/changing-fonts-in-matplotlib