Fix pyflakes and pep8 errors

Use extra asserts to use previously unused variables in tests,
such as `cass` and `response`.

Fix only pyflakes errors in docs/conf.py

tests/integration/test_urllib3.py

@@ -20,7 +20,7 @@ def scheme(request):
 @pytest.fixture(scope='module')
 def verify_pool_mgr():
     return urllib3.PoolManager(
-        cert_reqs='CERT_REQUIRED', # Force certificate check.
+        cert_reqs='CERT_REQUIRED',  # Force certificate check.
         ca_certs=certifi.where()
     )