Add upper-constraints.txt to releasenotes tox environment

Without these dependencies, the releasenotes build does not actually
work.

Change-Id: Iebb060857df6ec0b582e7635844e8505e09f9a4f

Author: 98k

tox.ini

@@ -58,5 +58,7 @@ import_exceptions = cloudkittyclient.i18n
 
 [testenv:releasenotes]
 basepython = python3
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+  -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html