Only run pep8 under python 3
Only run pep8 under python 3

The pep8 checks done for py3 are more stringent than those done for
py2, so it is sufficient to only run checks with py3. This changes
the pep8 job to use py3 and removes the pep8-py35 job.

This is consistent with other projects such as nova and nova-powervm.

Change-Id: I42661d80b1fef52c2a54eb3ee65d3f1ec4e8c306

powervm/pypowervm,pypowervm/pypowervm