Update client.py (#1697)

psycopg_pool updated and generated a RuntimeWarning. 
"RuntimeWarning: opening the async pool AsyncConnectionPool in the constructor is deprecated and will not be supported anymore in a future release. Please use `await pool.open()`, or use the pool as context manager using: `async with AsyncConnectionPool(...) as pool: `..."
update "create_pool" function, call "await pool.open()" explicitly.

tortoise/backends/psycopg/client.py

@@ -85,7 +85,9 @@ async def create_connection(self, with_db: bool) -> None:
             )
 
     async def create_pool(self, **kwargs) -> AsyncConnectionPool:
-        return AsyncConnectionPool(**kwargs)
+        pool = AsyncConnectionPool(open=False, **kwargs)
+        await pool.open()
+        return pool
 
     async def db_delete(self) -> None:
         try: