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

Commit 173b11cb50e115fe12baa097a05db36f24f2e181 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
1 file changed