CI: Clean up deps for various doc builds

- Define a single location for doc-build deps
  - As a side-effect, use constraints for api-ref builds
- Remove test-requirements.txt from those deps

Change-Id: If6cc8702e89f5110ad89ba933f55641de02550e9

tox.ini

@@ -121,14 +121,13 @@ basepython = python3
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
-  -r{toxinidir}/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:api-ref]
 # This environment is called from CI scripts to test and publish
 # the API Ref to docs.openstack.org.
 basepython = python3
-deps = -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 allowlist_externals =
   rm
 commands =
@@ -170,9 +169,7 @@ commands = bindep test doc
 
 [testenv:releasenotes]
 basepython = python3
-deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
-  -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]