Honor test-requirements.txt for pep8 checks

Setup requirements for pep8 checks according to specification
in test-requirements.txt. Is needed to pin hacking.

Change-Id: I6fb44a1e42181cb56e8206783c9681d7eb49e060

tox.ini

@@ -36,6 +36,8 @@ commands =
 skip_install = True
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  # NOTE(yoctozepto): this is to pin hacking to the tested version
+  -c{toxinidir}/test-requirements.txt
   hacking
 commands = flake8 {posargs}