Keep edge provenance (#94)

* Update kgx.py

* Update kgx.py

* Update kgx.py

can't delete edge keys while looping over them.

* just collect then update

* Update requirements.txt (#93)

dags/roger/models/kgx.py

@@ -478,6 +478,13 @@ def merge(self):
                     edges['subject'] + edges['predicate'] +
                     edges['object'] +
                     edges.get("biolink:primary_knowledge_source", ""))
+                keys_to_del = set()
+                for key in edges:
+                    if key.startswith('biolink:'):                        
+                        keys_to_del.add(key)
+                for k in keys_to_del:
+                    edges[k.replace('biolink:', '')] = edges[k]
+                    del edges[k]
                 stream.write(json.dumps(edges).decode('utf-8') + '\n')
 
         write_merge_metric['edges_writing_time'] = time.time() - start_edge_jsonl