Fix: Error while loading Contacts when PersonAccount is enabled (#3756)

W-15109524

With recent change for polymorphic data support, there is a bug
introduced while loading Contacts entity where PersonAccount is enabled

cumulusci/tasks/bulkdata/load.py

@@ -835,7 +835,7 @@ def _generate_contact_id_map_for_person_accounts(
 
                 # Join maps together to get tuple (Contact ID, Contact SF ID) to insert into step's ID Table.
                 if self._old_format:
-                    yield (contact_mapping.table + "-" + contact_id, contact_sf_id)
+                    yield (contact_mapping.table + "-" + str(contact_id), contact_sf_id)
                 else:
                     yield (contact_id, contact_sf_id)