Add upper contstraints to tox doc env

This is to avoid using untested package versions.
See for example recent fix for the sphinx that
broke openstack-tox-docs job

https: //review.opendev.org/c/openstack/openstack-helm-infra/+/892376

Change-Id: I78aeb94c02831d5a91fd5308324b55e534b38376

tox.ini

@@ -14,7 +14,9 @@ passenv = *_proxy,*_PROXY
 commands = {posargs}
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build
   sphinx-build -W -b html doc/source doc/build/html