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: Ice3cffa36e9167f2f1f30337372cf9187af5e02e

oslo_concurrency/processutils.py

@@ -575,9 +575,9 @@ def ssh_execute(ssh, cmd, process_input=None,
             # to decode it, or an Unicode string otherwise. Encode to the
             # default encoding (ASCII) because mask_password() decodes from
             # the same encoding.
-            if isinstance(stdout, unicode):
+            if isinstance(stdout, six.text_type):
                 stdout = stdout.encode()
-            if isinstance(stderr, unicode):
+            if isinstance(stderr, six.text_type):
                 stderr = stderr.encode()
         else:
             # fsencode() is the reverse operation of fsdecode()

tox.ini

@@ -14,6 +14,7 @@ commands =
     env TEST_EVENTLET=1 lockutils-wrapper python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
+basepython = python3
 deps =
   -r{toxinidir}/test-requirements.txt
 commands =