Use implicit namespace packages
This MR removes the explicit declaration of namespace_packages = ligo
in favour of implicit namespace packages, see https://peps.python.org/pep-0420/. This resolves an issue seen in the conda-forge builds where the installed ligo-followup-advocate 1.1.10 package could not be tested via python -m pytest --pyargs ligo.followup_advocate.test
because of a relative import failure.