stdlib: add argument default values (#9501)

stdlib/datetime.pyi

@@ -261,7 +261,7 @@ class datetime(date):
     def utcfromtimestamp(cls: type[Self], __t: float) -> Self: ...
     if sys.version_info >= (3, 8):
         @classmethod
-        def now(cls: type[Self], tz: _TzInfo | None = ...) -> Self: ...
+        def now(cls: type[Self], tz: _TzInfo | None = None) -> Self: ...
     else:
         @overload
         @classmethod