Fix tox -e venv -- reno new <slug>

Change I7e8c47dead1e019e8705db3ff7559dd39b1d90d9 broke
the ability to create a new release note from the venv
tox target because the reno requirement was moved from
test-requirements.txt. This fixes it.

Change-Id: Ifac83121388728fa445e0ed4433c9d981c057737

tox.ini

@@ -87,6 +87,10 @@ commands =
     {toxinidir}/openstackclient/tests/functional/run_stestr.sh {posargs}
 
 [testenv:venv]
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/doc/requirements.txt
 commands = {posargs}
 
 [testenv:cover]