As a User, I can create a PythonPackageContent from an Artifact uploaded to Pulp
The upload workflow in Pulp 3 is:
- Use pulpcore REST API to upload bits and create an Artifact
- POST to a Detail ContentUnit (api/v3/content/python/) to create a PythonPackageContent from a specified Artifact
This story is to make sure that (2) works. (1) Does not require anything from the plugin.
In Pulp 2, we create Uploaded Python ContentUnits here  by using twine, but we should move away from doing it that way.  We only use a small part of twine (which isn't really supposed to be used as a library), so we will need to implement that functionality. 
Extra credit: Put at least 1 of each upload type in https://github.com/PulpQE/pulp-fixtures (half extra credit, provide a link to 1 of each here)