Add ability to upload to pypi test server

[timburke]

There is already a way to end-to-end test the release process minus actually uploading the package by tagging the release using test@<tag-name>. However this does not test the actual twine upload process, which could fail due to an installed package issue. There should be an option for public python package like stage@<tag-name> that does a complete closed loop test to the test_pypi index.