Add dependencies to releasenotes environment
The current tox 'releasenotes' environment does not include the
doc/requirements.txt as a dependency, and as a result sphinx-build
cannot be found when attempting to build the release notes.  So add
it.
@@ -90,6 +90,7 @@ whitelist_externals =
make
[testenv:releasenotes]
+deps = {[testenv:docs]deps}
commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
[flake8]
