CI: Modify publish.yml to support TestPyPI

[ColmTalbot]

Updated the GitHub Actions workflow to trigger on push events and added a new job for publishing to TestPyPI.

Closes #978

[ColmTalbot]

Looks like this is successfully pushing now, it will attempt to push to test.pypi for any push to bilby-dev/bilby. We should be more careful about people making branches on this fork to avoid overloading testpypi. If there is too much of a load on testpypi and we can't reduce the number of pushes, we can try setting up a different upload point.

[mj-will]

@ColmTalbot does this mean the PyPI uploads now happen when the tag is made rather than when the release is made? If so, we should update the docs.

[ColmTalbot]

does this mean the PyPI uploads now happen when the tag is made rather than when the release is made? If so, we should update the docs.

I guess technically yes, I can make the check if: github.event_name == 'release' && github.event.action == 'published' to keep the same behaviour. We should really be doing them together though.

[mj-will]

I guess technically yes, I can make the check if: github.event_name == 'release' && github.event.action == 'published' to keep the same behaviour. We should really be doing them together though.

Perhaps we should just remove the documentation for making tags from the terminal, that way we guide people towards always making them together?

[ColmTalbot]

Workflow needs to be triggered on release.