Fix `tox -epep8` for python3.8 users

For those who have default python3 pointing to 3.8
an error occurs for `tox -epep8` as it tries to
use older pyflakes that has bug when running under
python3.8.

Bump `hacking` to later versions where the bug
is fixed.

Change-Id: I7bdde5c67b41be2c9fed92e2713ef22399013169

test-requirements.txt

@@ -4,5 +4,5 @@
 
 coverage>=3.6
 doc8
-hacking>=1.1.0,<1.2.0 # Apache-2.0
+hacking>=3.0.1,<3.1.0 # Apache-2.0
 requests-mock>=1.0