Update to sagemath 8.4.
    
    Also:
    - Build for python 3 instead of python 2 due to upcoming python 2 removal.
    - Add -python3 and -escape patches to fix problems with python 3.
    - Drop -nofstring patch, only needed for python 2.
    - Drop upstreamed -eclib patch.
    - Switch from atlas to openblas and rename -atlas patch to -openblas.
    - Add -buildroot patch and only build cython interfaces once.