Use lowercase tuple where possible (#6170)

stdlib/poplib.pyi

@@ -26,7 +26,7 @@ class POP3:
     def set_debuglevel(self, level: int) -> None: ...
     def user(self, user: str) -> bytes: ...
     def pass_(self, pswd: str) -> bytes: ...
-    def stat(self) -> Tuple[int, int]: ...
+    def stat(self) -> tuple[int, int]: ...
     def list(self, which: Any | None = ...) -> _LongResp: ...
     def retr(self, which: Any) -> _LongResp: ...
     def dele(self, which: Any) -> bytes: ...