Only build scipy once (per Python)
    
    The build_sphinx setup.py command previously used to build the docs
    calls build_ext --inplace internally. That's one build.
    
    Later the install command was called and that calls build as well.
    Inplace and non-inplace builds happen separately.
    
    By first explicitly building with build and then invoking sphinx
    trough the Makefile, we can use install --skip-build (%pyX_install).
    
    Explicit call to sphinx also removes the need to remove conf.py
    from bundled projects.
    
    This also saves us setting the build flags in %install, which was
    previously only needed because of the build happening there.
    
    While doing this, I've also switched the build section to build for
    for Python 3 and Python 2 in a loop to make the specfile DRY.