psycopg2: Fix the return type of Composable.as_string (#7984)

Signed-off-by: Zixuan James Li <359101898@qq.com>

stubs/psycopg2/psycopg2/sql.pyi

@@ -2,7 +2,7 @@ from typing import Any
 
 class Composable:
     def __init__(self, wrapped) -> None: ...
-    def as_string(self, context) -> None: ...
+    def as_string(self, context) -> str: ...
     def __add__(self, other): ...
     def __mul__(self, n): ...
     def __eq__(self, other): ...