API REF docs fails on `exception: No module named 'os_openapi')`

[lpiwowar]

• I ran the building of the image with the API docs locally, and it is failing for me on the following error (on main as well -> not caused by this PR). It might be related to the error we are seeing in the CI for the upstream docs though. Somebody should take a look on this.

$ make build-image-os BUILD_OCP_DOCS=true FLAVOR=gpu BUILD_GPU_ARGS="--device nvidia.com/gpu=all" OS_API_DOCS=true OS_PROJECTS="keystone"
...
+ tox -etext-api-ref
text-api-ref: install_deps> python -I -m pip install os-api-ref -r /tmp/os_docs_temp/keystone/doc/requirements.txt -c https://releases.openstack.org/constraints/upper/2025.2
.pkg: _optional_hooks> python /opt/app-root/lib64/python3.11/site-packages/pyproject_api/_backend.py True pbr.build
.pkg: get_requires_for_build_editable> python /opt/app-root/lib64/python3.11/site-packages/pyproject_api/_backend.py True pbr.build
.pkg: build_editable> python /opt/app-root/lib64/python3.11/site-packages/pyproject_api/_backend.py True pbr.build
text-api-ref: install_package_deps> python -I -m pip install 'Flask!=0.11,>=1.0.2' 'Flask-RESTful>=0.3.5' 'PyJWT>=1.6.1' 'SQLAlchemy>=1.4.0' 'WebOb>=1.7.1' 'Werkzeug>=0.15.0' 'bcrypt>=3.1.3' 'cryptography>=2.7' 'dogpile.cache>=1.0.2' 'jsonschema>=3.2.0' 'keystonemiddleware>=7.0.0' 'msgpack>=0.5.0' 'oauthlib>=0.6.2' 'oslo.cache>=1.26.0' 'oslo.config>=6.8.0' 'oslo.context>=2.22.0' 'oslo.db>=6.0.0' 'oslo.i18n>=3.15.3' 'oslo.log>=3.44.0' 'oslo.messaging>=5.29.0' 'oslo.middleware>=3.31.0' 'oslo.policy>=4.5.0' 'oslo.serialization!=2.19.1,>=2.18.0' 'oslo.upgradecheck>=1.3.0' 'oslo.utils>=3.33.0' 'osprofiler>=1.4.0' 'pbr!=2.1.0,>=2.0.0' 'pycadf!=2.0.0,>=1.1.0' 'pysaml2>=5.0.0' 'python-keystoneclient>=3.8.0' 'scrypt>=0.8.0' 'stevedore>=1.20.0'
text-api-ref: install_package> python -I -m pip install --force-reinstall --no-deps /tmp/os_docs_temp/keystone/.tox/.tmp/package/2/keystone-0.0.0-0.editable-py3-none-any.whl
text-api-ref: commands[0]> sphinx-build --keep-going -j auto -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
Running Sphinx v8.1.3
loading translations [en]... done
[openstackdocstheme] version: 3.5.0
[openstackdocstheme] connecting html-page-context event handler

Extension error:
Could not import extension os_openapi (exception: No module named 'os_openapi')
text-api-ref: exit 2 (0.57 seconds) /tmp/os_docs_temp/keystone> sphinx-build --keep-going -j auto -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html pid=1709
.pkg: _exit> python /opt/app-root/lib64/python3.11/site-packages/pyproject_api/_backend.py True pbr.build
  text-api-ref: FAIL code 2 (29.07=setup[28.50]+cmd[0.57] seconds)
  evaluation failed :( (29.19 seconds)
+ echo 'WARNING: API-Ref build failed for keystone'
WARNING: API-Ref build failed for keystone
+ exit 1

Originally posted by @lpiwowar in #83 (review)