more-itertools dropped Python 2 support with 6.0. The recent upgrade (#5291)
to 7.0 results in the package no longer working under Python 2 and breaks
packages depending on it like pytest.
Split the PKGBUILD up into a Python 2 (5.0) and Python 3 (7.0) version.
Add an epoch to the Python 2 version so that package upgrades work.