Fix deps for using venv tox target to create a release note

The reno package dependency is in doc/requirements.txt which
needs to be in the tox venv target deps if you want to create
a release note using:

  tox -e venv -- reno new <slug>

Change-Id: Ifa1fd55b2c8a3f65a74d9e5e2f0482d015c97723

tox.ini

@@ -51,6 +51,11 @@ commands = flake8 {posargs}
 
 [testenv:venv]
 commands = {posargs}
+# The docs requirements are included here for creating release notes, e.g.:
+#   tox -e venv -- reno new <slug>
+deps =
+  {[testenv]deps}
+  -r{toxinidir}/doc/requirements.txt
 
 [testenv:cover]
 envdir = {toxworkdir}/shared