Only require Mock on Python 3.4 and older. Use the builtin one elsewhere.
Mock 2.0 is using pbr which is hostile to environments without network access.
It's not required on Python 3.5+ so I made it possible to use unittest.mock
on this version.
Updated tox.ini to reflect this. Tested with 2.7.11, 3.3.6, 3.4.5, and 3.5.2 on macOS 10.12.1 with tox and pyenv.