Add TestPyPI GHA

[philiporlando]

Create a separate action/workflow that is kicked off manually. That way you could publish a version to Test PyPI whenever you thought was the right time rather than on tagging a version or every time you push to your main branch. You can't overwrite versions on PyPI, so for something like that I might recommend adding -pre1 or something to the version so you can could publish to Test PyPI multiple times for the same version number.

The docs for writing a workflow that you can trigger manually are over here:

https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#workflow_dispatch