Upgrade Peewee to latest 3.x

This requires a number of small changes in the data model code, as well as additional testing.

data/model/test/test_repository.py

@@ -8,6 +8,7 @@ from data.model.repository import create_repository, purge_repository, is_empty
 from data.model.repository import get_filtered_matching_repositories
 from test.fixtures import *
 
+
 def test_duplicate_repository_different_kinds(initialized_db):
   # Create an image repo.
   create_repository('devtable', 'somenewrepo', None, repo_kind='image')