python: remove gob usage

This will become problematic when tinkering with
`(claircore.Package).Kind`. Just having gigantic fixtures seems to be
the best solution while the JSON serialization is load-bearing.

Change-Id: I3ac5bf8914c178d4fa5b14e69155860762155c21
Signed-off-by: Hank Donnay <hdonnay@redhat.com>

Author: hdonnay