Updates for matplotlib>3.=1

[aperezhortal]

• [x] Fix OSX installation
• [x] Update package to work with matplotlib>=3.1
• [x] Update required packages (matplotlib>=3.1)
• [x] Run black
• [x] Remove cpp sources. Now cython is strictly needed.

[mhaberler]

works for me using Python 3.8.6 and matplotlib 3.3.3

after a brew update/upgrade , gcc and g++ are now available as gcc-10 and g++-10 so I used

export CC=gcc-10
export CXX=g++-10

other than that, good to go

thanks a lot!