Python Support

Steps I followed to find the issue

1. Create an empty python repository
2. Create a remote with one or more packages in include (I used shelf-reader) and have policy be on_demand
3. Sync repo with remote
4. Try to create publication with latest repo version

I got the following error traceback:

"description": "'NoneType' object has no attribute 'sha256'",
        "traceback": "  File \"/usr/local/lib/pulp/lib64/python3.7/site-packages/rq/worker.py\", line 886, in perform_job\n    rv = job.perform()\n  File \"/usr/local/lib/pulp/lib64/python3.7/site-packages/rq/job.py\", line 664, in perform\n    self._result = self._execute()\n  File \"/usr/local/lib/pulp/lib64/python3.7/site-packages/rq/job.py\", line 670, in _execute\n    return self.func(*self.args, **self.kwargs)\n  File \"/home/vagrant/devel/pulp_python/pulp_python/app/tasks/publish.py\", line 65, in publish\n    write_simple_api(publication)\n  File \"/home/vagrant/devel/pulp_python/pulp_python/app/tasks/publish.py\", line 128, in write_simple_api\n    checksum = content_artifact.artifact.sha256\n"