Debian binary packages don't normally include the test suite. Some Python
library developers are treating the pypi releases in a similar way, as if
they're just for deployment. They think anybody who needs the test suite is
doing development and will clone from the github repo.
With my upstream developer hat on: source packages on PyPI are meant for end users to install via pip. They often include generated artifacts, and don't include things that aren't intended for installation via pip (tests being just one of these).
For distribution packaging purposes, the GitHub tags are generally preferrable. GitHub makes archives of tagged releases available as tarballs, so this is generally a simple tweak to debian/watch.