Rename python package from systemml-*-python.tgz to systemml-*-python.tar.gz

I encountered this issue because pypi has migrated to a new process: https://packaging.python.org/guides/migrating-to-pypi-org/#uploading

As noted in the above document, the recommended way to upload python packages to pypi is now via `twine`. However, if we use `twine` with our current package naming scheme (i.e. tgz), then it complains `ValueError: Unknown distribution format: 'systemml-0.15.0-python.tgz'`. Hence, I would recommend to use suffix `tar.gz` in our subsequent releases. This way we also are compatible with default package naming convention of pypi: `tar.gz`.

acs_s gweidner deron dusenberrymw reinwald Suggestions ? Any takers ?