[CI] auto-publish to PyPI

[hydrosquall]

Once this is stable, this would prevent the need to manually re-publish. I have a similar file which uploads the wheels for any new tagged releases.

https://github.com/hydrosquall/tiingo-python/blob/master/.travis.yml

Travis is one possible provider that's free for open source, but I don't have particular attachment to it (github actions, azure pipelines, circleci etc all could work too).

https://docs.travis-ci.com/user/deployment/pypi/

[domoritz]

I heard rumors that Streamlit might adopt our approach, in which case this repo can just serve as a POC. Let's see how things develop over the next weeks and then re-evaluate. Until then, we can make manual releases.