Correct use session during create db object.

This enable python3.8 support for staffeln
diff --git a/staffeln/db/sqlalchemy/api.py b/staffeln/db/sqlalchemy/api.py
index 6d9325f..ff2fcfd 100644
--- a/staffeln/db/sqlalchemy/api.py
+++ b/staffeln/db/sqlalchemy/api.py
@@ -211,12 +211,16 @@
         return obj
 
     def _create(self, model, values):
-        obj = model()
-        cleaned_values = {
-            k: v for k, v in values.items() if k not in self._get_relationships(model)
-        }
-        obj.update(cleaned_values)
-        obj.save()
+        session = get_session()
+        with session.begin():
+            obj = model()
+            cleaned_values = {
+                k: v
+                for k, v in values.items()
+                if k not in self._get_relationships(model)
+            }
+            obj.update(cleaned_values)
+            obj.save(session=session)
         return obj
 
     @staticmethod