Add return type for tkinter.Text.mark_previous (#4986)

Author: Akuli

stdlib/tkinter/__init__.pyi

@@ -2503,7 +2503,7 @@ class Text(Widget, XView, YView):
     def mark_set(self, markName: str, index: _TextIndex) -> None: ...
     def mark_unset(self, *markNames: str) -> None: ...
     def mark_next(self, index: _TextIndex) -> Optional[str]: ...
-    def mark_previous(self, index: _TextIndex): ...
+    def mark_previous(self, index: _TextIndex) -> Optional[str]: ...
     # **kw of peer_create is same as the kwargs of Text.__init__
     def peer_create(self, newPathName: Union[str, Text], cnf: Dict[str, Any] = ..., **kw: Any) -> None: ...
     def peer_names(self) -> Tuple[_tkinter.Tcl_Obj, ...]: ...