Set download_url in setup.py.

This appears to now be required by either PyPI or the gh-action-pypi-publish
github action. When not set, PyPI now returns a 400 error:

400 '' is not a valid url. See
https://packaging.python.org/specifications/core-metadata for more information.

The server could not comply with the request since it is either malformed or
otherwise incorrect.

'' is not a valid url. See
https://packaging.python.org/specifications/core-metadata for more information.

python/setup.py

@@ -78,6 +78,7 @@ def find_version(*file_paths):
         'Topic :: Software Development :: Libraries :: Python Modules',
     ],
     url='https://github.com/PointOneNav/fusion-engine-client',
+    download_url=f'https://github.com/PointOneNav/fusion-engine-client/archive/refs/tags/v{version}.tar.gz',
     packages=find_packages(where='.'),
     entry_points={
         'console_scripts': [