Python 2.7 is deprecated now, and some dependencies have dropped their
support for it. We should also just drop support for it rather than
pinning the dependency versions.
This commit also switches to a generic Python 3 tox environment,
adds additional test jobs for more Python 3 versions, and cleans up
some Python 2 compatibility cruft.
Change-Id: I3e7cc1713dec1bada4a85f75362c4fa4f89ceae6
647bb9c660