Is there a place where we can figure out which constructs are supported in pypi? We have some html in the readme

https://github.com/pypa/warehouse is the place to open/read issues on these topics.

You can always push a trial version to the testpypi server with python3 -m twine upload --repository testpypi dist/*. This will go to a test server instead of the public PyPI so you can see if/how the HTML renders. Also, check the relative links in markdown. These might need to be converted to absolute links to work correctly on PyPI.

My first commit tried to add CHANGES.md to the end but that file is not available.

Thanks! 👍

Which repo contains the Sentry web user interface or is that not open source?

That's getsentry/sentry. I think you already contributed to it :)