refactor: Move hacking dependency from tox.ini to test-reqs

This dependency was moved from test-requirements.txt to tox.ini[deps] a
long time ago: I0111c41bea6a6caf5ffba1f5c34489854d9c9747 due to some pip
resolver issues related to lower-constraints.txt

Since then, a lot has changed and this probably doesn't apply anymore
(other projects like nova already keep the dependency in
test-requirements.txt).

We actually import hacking from neutron.tests.unit (because we have
custom rules), so it makes more sense to keep the dependency with the
rest of requirements.

Change-Id: I02ad072fb2982f6968db987cb822efdc1b6f10ba

test-requirements.txt

@@ -1,3 +1,4 @@
+hacking>=6.1.0,<6.2.0 # Apache-2.0
 coverage>=4.4.1 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 python-subunit>=1.0.0 # Apache-2.0/BSD

tox.ini

@@ -28,7 +28,6 @@ deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
-  hacking>=6.1.0,<6.2.0 # Apache-2.0
 allowlist_externals = bash
 commands =
   bash {toxinidir}/tools/pip_install_src_modules.sh "{toxinidir}"