Fix being able to create a reno using tox -e venv

Iace251446bafbd8963bc5a073de50ade583f6e46 moved the
reno dependency to a new requirements file which broke
the ability to create a release note using:

  tox -e venv -- reno new <slug>

This fixes it by adding the doc/requirements into the
venv tox environment.

Change-Id: I243d4d5148964511a83855d2b1b451aa1543b98b

tox.ini

@@ -29,6 +29,11 @@ commands = flake8 {posargs}
 commands = bandit -r novaclient -n5 -x tests
 
 [testenv:venv]
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/doc/requirements.txt
 commands = {posargs}
 
 [testenv:docs]