Create function to download python packages

[kdelee]

We need a function to download published python packages for test_restore.py

It is my opinion that this should not depend on pip but rather be able to parse whatever metadata that the python repo provides about its structure and form its request based on that.

[jeremy-audet]

If such a function was written for Pulp Smash, it would be OK to use that here in pulp-migrate to solve this issue.