There's no particular reason why every Python package has to distribute the Debian orig tarball via PyPI, though. *My* projects are almost all on GitHub these days, so I offer the git tags and github archives as an option to get the "full source"; in the past, I have hosted projects elsewhere, and released tarballs elsewhere, and there is of course nothing wrong with this.
If you want to make your PyPI sdists do double-duty, there's usually nothing terribly wrong with this, but given that they must include generated artifacts (like .c source generated by Cython) in order to work well for pip, and pip is typically how people consume PyPI, I don't view these as the ideal "upstream source" for Debian.
And of course, there are extreme examples like cryptography: The python-cryptography source package is 384K, the python-cryptography-vectors source package (necessary for running the cryptography tests) is 26MB!
FWIW: I should note that I'm a fan of having my tests as a subpackage of the main package (ie. mything.tests) so they can be run by an end-user to verify that an installed copy of the package is actually working. I do this in most of the projects I'm upstream for; so the tests are included in my sdist, but other files are not (like documentation).