Try changing read_sql_to_dataframe to take a engine instead of a connection, to see if this will help with unclosed connections to the sql database

cdplib/db_handlers/SQLHandler.py

@@ -511,7 +511,7 @@ class SQLHandler:
             connection.close()
             '''
             data = pd.read_sql(sql=query,
-                               con=self.engine,
+                               con=self._engine,
                                **read_sql_kwargs)
             return data