Add mock as a dependency for py27 and pypy tox environments
Add mock as a dependency for py27 and pypy tox environments

tox won't install packages listed in tests_require, but not in
test-requirements.txt. tests_require packages get installed by
setuptools test command only.

Kuniwak/vint,RianFuro/vint,RianFuro/vint,Kuniwak/vint