Issue #295: use a setup_requires dependency
Issue #295: use a setup_requires dependency

Rather than hand-checking, let setuptools do the checking. This avoids
issues with distutils being unable to parse some setuptools release
versions, which could bite us in future.

nett55/mock,frankier/mock,testing-cabal/mock,lord63-forks/mock,OddBloke/mock,rbtcollins/mock