sqlalchemy 2 compatibility

- avoid backref warnings by adding objects to session explicitly before creating any relationships
- remove unnecessary `[]` around scalar query
- use `text()` wrapper on connection.execute
- engine.execute is removed
- update import of declarative_base
- ensure RemovedIn20Warning is available for warnings filters on sqlalchemy < 1.4 (needs editable install to avoid pytest path mismatch)
- explicitly relay password in engine.url to alembic

jupyterhub/user.py

@@ -416,9 +416,10 @@ class User:
                 yield orm_spawner
 
     def _new_orm_spawner(self, server_name):
-        """Creat the low-level orm Spawner object"""
-        orm_spawner = orm.Spawner(user=self.orm_user, name=server_name)
+        """Create the low-level orm Spawner object"""
+        orm_spawner = orm.Spawner(name=server_name)
         self.db.add(orm_spawner)
+        orm_spawner.user = self.orm_user
         self.db.commit()
         assert server_name in self.orm_spawners
         return orm_spawner