Python Support

The upload workflow in Pulp 3 is:

1. Use pulpcore REST API to upload bits and create an Artifact
2. 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 [0] by using twine, but we should move away from doing it that way. [1] 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. [2]

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)

[0]: https://github.com/pulp/pulp_python/blob/master/plugins/pulp_python/plugins/models.py#L125-L150
[1]: https://pulp.plan.io/issues/3157
[2]: https://github.com/pypa/twine/blob/3efd3b5d2715f537096b95cb2e6853e02990b059/twine/package.py