gh-140609: delete unused _stdlib_enums in enum.py (#140618)

Signed-off-by: yihong0618 <zouzou0208@gmail.com>

Lib/enum.py

@@ -17,7 +17,7 @@ __all__ = [
 # Dummy value for Enum and Flag as there are explicit checks for them
 # before they have been created.
 # This is also why there are checks in EnumType like `if Enum is not None`
-Enum = Flag = EJECT = _stdlib_enums = ReprEnum = None
+Enum = Flag = EJECT = ReprEnum = None
 
 class nonmember(object):
     """
@@ -2189,5 +2189,3 @@ def _old_convert_(etype, name, module, filter, source=None, *, boundary=None):
         members.sort(key=lambda t: t[0])
     cls = etype(name, members, module=module, boundary=boundary or KEEP)
     return cls
-
-_stdlib_enums = IntEnum, StrEnum, IntFlag