Fix creating release notes using tox -e venv -- reno new <slug>

Commit 173b11cb50 moved the reno
dependency from test-requirements to doc/requirements which weren't
used by the venv tox target, which means you can't create release
notes the easy way "tox -e venv -- reno new <slug>". This fixes it
by adding the doc/requirements into the venv target.

Change-Id: Ic3724435176eab6f7fcc53cf5379f7eee44baa71

tox.ini

@@ -123,6 +123,10 @@ commands =
     tempest run --serial --regex '\[.*\bsmoke\b.*\]' {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:venv-tempest]