Revert "manifold.py: don't use ABCMeta as metaclass"
Revert "manifold.py: don't use ABCMeta as metaclass"

This reverts commit 811ba1324047ed6decf043828d4668a81ec3d68f.

pymanopt/pymanopt,nkoep/pymanopt,nkoep/pymanopt,nkoep/pymanopt,pymanopt/pymanopt