[CQT-88] Add a Publish to TestPyPI step to the release workflow

[rturrado]

Publishing to TestPyPI should catch errors in deployment a lot quicker.

• Just before publishing to PyPI we should publish to TestPyPI.
• The event triggering the publishing to TestPyPI is exactly the same as the event triggering the publishing to PyPI (when you create a new release in GitHub).
• If publishing to TestPyPI works, publishing to PyPI should also work.
• If publishing to TestPyPI fails, we avoid publishing to PyPI.