Use upper-constraint in doc generation

Currently doc jobs are broken because the latest sphinx 7.2.0 is not
compatible with the latest release of openstackdocstheme.

This updates the docs/releasenotes target to use the global upper
constraints to avoid that issue.

Change-Id: I8aeae9a380c4560c52bc13abd1745d69845071a0

tox.ini

@@ -22,7 +22,9 @@ commands = {posargs}
 [testenv:docs]
 allowlist_externals =
   rm
-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 -fr doc/build
   sphinx-build -W --keep-going -b html doc/source doc/build/html
@@ -43,7 +45,7 @@ import_exceptions = oslo_middleware._i18n
 [testenv:releasenotes]
 allowlist_externals =
   rm
-deps = -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html