Add Test PyPI action to GitHub Publishing Workflow

[jaymedina]

Closes #102

• Successful upload to Test PyPI with Twine instead of OpenAstronomy action
• Trigger run after being labeled publish-to-test
• Incrementally increase version number based on what already exists in Test PyPI

[codecov]

Codecov Report

All modified lines are covered by tests ✅

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[jaymedina]

Let me know what you think of the versioning for this @falkben . Ideally, I would have liked to keep the versioning syntax that's used in the official pypi releases, but that seemed tricky to do, although I didn't explore the solutions enough.

I went with defining each version based on the date/time of the current run, which is simple. This is what a release looks like on Test PyPI:

Users CAN choose to download from Test PyPI so I wonder if we should keep our version numbers the same as how we write them in PyPI.

Edit: David suggests versioning with commit hash, and mentions that it can also point directly to the latest commit before the release to Test PyPI took place, so this sounds like it would be very useful.