Switch pep8 job to python 3
Switch pep8 job to python 3

pep8 under python 3 is more strict than under python 2, so to make
sure our projects meet the more strict standards we want to run the
pep8 jobs using python 3 by default.

Change-Id: I7b2a817147a0f1d1c7aabe09161a4df2ebec503f

openstack/oslo.context