Correct sre_parse.State.opengroup (#6608)

Author: AlexWaygood

stdlib/sre_parse.pyi

@@ -24,7 +24,7 @@ class _State:
     def __init__(self) -> None: ...
     @property
     def groups(self) -> int: ...
-    def opengroup(self, name: str = ...) -> int: ...
+    def opengroup(self, name: str | None = ...) -> int: ...
     def closegroup(self, gid: int, p: SubPattern) -> None: ...
     def checkgroup(self, gid: int) -> bool: ...
     def checklookbehindgroup(self, gid: int, source: Tokenizer) -> None: ...