Fix flake8 3.6.0 errors

Flake8 3.6.0 now warns about both line break after and *before* binary
operator, you have to choose whether you use W503 or W504. Disable the
newer W504.

Also, ignore warning about invalid escape sequence in regex.

Change-Id: Ibfbd5df21e01d5a7bd44a216ff63bc805dd5c186

roles/add-build-sshkey/library/test_sshagent_remove_keys.py

@@ -78,7 +78,7 @@ class TestAgent(testtools.TestCase):
         l = a.list()
         self.assertEqual(2, len(l))
 
-        run('^(?!\(stdin\)).*')
+        run('^(?!\(stdin\)).*')  # noqa
 
         l = a.list()
         self.assertEqual(1, len(l))

tox.ini

@@ -53,6 +53,8 @@ commands = {posargs}
 # These are ignored intentionally in openstack-infra projects;
 # please don't submit patches that solely correct them or enable them.
 # E402 - ansible modules put documentation before imports. Align to ansible.
-ignore = E125,E129,E402,E741,H
+# W504 - line break after binary operator, we cannot have both
+#        W503 and W504 enabled
+ignore = E125,E129,E402,E741,W504,H
 show-source = True
 exclude = .venv,.tox,dist,doc,build,*.egg