Add action to publish to PyPI

[jrbourbeau]

cc @jacobtomlinson if you get a chance to take a look at this

xref #25

[jrbourbeau]

That's a good point, I've added the same if statement to the step that builds the dist. I also removed the github.event_name == 'push' from the if statements as we've specified with

on:
  push:
    branches: master

that the workflow should only run when pushing to master

[jrbourbeau]

Thanks for reviewing @jacobtomlinson!