Limit where dnspython3 is installed

tempest is still testing tools that have python2 environments, and due to
the branchless setup, we need to make sure dnspython3 is not installed on
python2.7

Change-Id: I8e42ddd73c4c4f4a2d5cc348b75ddf342c7d04f2
Signed-off-by: Graham Hayes <gr@ham.ie>
1 file changed