Add manual trigger to deploy to pypi

[abidlabs]

This creates a "run workflow" button for the "deploy to pypi" action, similar to this:

I haven't been able to fully test the deployment on my fork because secrets, etc. don't transfer over to the fork, but I can confirm that the button appears. I'll do a complete test once this is merged in.

Also removes the whole "merge to a branch starting with 'release'" to deploy a test branch as it's quite cumbersome and we won't need it if this works

Fixes: #2190

[freddyaboulton]

Thanks @abidlabs ! Only pending change is checking whether or not the version exists via pypi in the github action.

[abidlabs]

Added the pypi check near the beginning of the GitHub action. Thanks for the suggestion @freddyaboulton, and this should be good to go now.