Roundup doesn't generate sphinx-argparse docs

[nutjob4life]

🐛 Describe the bug

@collinss-jpl reports that release/2.1.2 of the pds-doi-serivce worked fine but https://nasa-pds.github.io/doi-service/api/index.html didn't include any argparse docs.

This could be that Roundup is executing sphinx-build in its own context instead of in the rounded-up package's venv. Although the preparation step does set the PATH appropriately so that if there's a sphinx-build in the venv it should be happening. Odd.

🕵️ Expected behavior

sphinx-argparse as well as any other Sphinx add-ons (like sphinxcontrib-napoleon) should be supported.

🦄 Related requirements

⚙️ Engineering Details

[collinss-jpl]

@nutjob4life Just combed over the Roundup log and noticed this here: https://github.com/NASA-PDS/doi-service/runs/4810121150?check_suite_focus=true#step:6:527

DEBUG:pds.roundup.util:Stderr = «/github/workspace/docs/source/api/index.rst:9: WARNING: Failed to import "create_parser" from "pds_doi_service.core.actions.action".
No module named 'pds_doi_service'
/github/workspace/docs/source/api/index.rst:17: WARNING: Failed to import "create_cmd_parser" from "pds_doi_service.core.util.initialize_production_deployment".
No module named 'pds_doi_service'

Could this be as simple as making sure the local checkout of the service is added to Roundup's PYTHONPATH?