set default python to python3
set default python to python3

Set the default python to python3 except for the py27 environment. We
have to set that explicitly to override the new default.

Change-Id: Ia540b70e648d585aa0de017f72ceb96214b67e27
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/stevedore