[3.9] bpo-42567: [Enum] call __init_subclass__ after members are added (GH-23714) (GH-23772)

When creating an Enum, `type.__new__` calls `__init_subclass__`, but at that point the members have not been added.

This patch suppresses the initial call, then manually calls the ancestor `__init_subclass__` before returning the new Enum class.
(cherry picked from commit 6bd94de168)

Lib/enum.py

@@ -9,6 +9,14 @@
         ]
 
 
+class _NoInitSubclass:
+    """
+    temporary base class to suppress calling __init_subclass__
+    """
+    @classmethod
+    def __init_subclass__(cls, **kwds):
+        pass
+
 def _is_descriptor(obj):
     """
     Returns True if obj is a descriptor, False otherwise.
@@ -176,7 +184,7 @@ def __prepare__(metacls, cls, bases):
                     )
         return enum_dict
 
-    def __new__(metacls, cls, bases, classdict):
+    def __new__(metacls, cls, bases, classdict, **kwds):
         # an Enum class is final once enumeration items have been defined; it
         # cannot be mixed with other types (int, float, etc.) if it has an
         # inherited __new__ unless a new __new__ is defined (or the resulting
@@ -211,8 +219,22 @@ def __new__(metacls, cls, bases, classdict):
         if '__doc__' not in classdict:
             classdict['__doc__'] = 'An enumeration.'
 
+        # postpone calling __init_subclass__
+        if '__init_subclass__' in classdict and classdict['__init_subclass__'] is None:
+            raise TypeError('%s.__init_subclass__ cannot be None')
+        # remove current __init_subclass__ so previous one can be found with getattr
+        new_init_subclass = classdict.pop('__init_subclass__', None)
         # create our new Enum type
-        enum_class = super().__new__(metacls, cls, bases, classdict)
+        if bases:
+            bases = (_NoInitSubclass, ) + bases
+            enum_class = type.__new__(metacls, cls, bases, classdict)
+            enum_class.__bases__ = enum_class.__bases__[1:] #or (object, )
+        else:
+            enum_class = type.__new__(metacls, cls, bases, classdict)
+        old_init_subclass = getattr(enum_class, '__init_subclass__', None)
+        # and restore the new one (if there was one)
+        if new_init_subclass is not None:
+            enum_class.__init_subclass__ = classmethod(new_init_subclass)
         enum_class._member_names_ = []               # names in definition order
         enum_class._member_map_ = {}                 # name->value map
         enum_class._member_type_ = member_type
@@ -324,6 +346,9 @@ def __new__(metacls, cls, bases, classdict):
             if _order_ != enum_class._member_names_:
                 raise TypeError('member order does not match _order_')
 
+        # finally, call parents' __init_subclass__
+        if Enum is not None and old_init_subclass is not None:
+            old_init_subclass(**kwds)
         return enum_class
 
     def __bool__(self):
@@ -701,6 +726,9 @@ def _generate_next_value_(name, start, count, last_values):
         else:
             return start
 
+    def __init_subclass__(cls, **kwds):
+        super().__init_subclass__(**kwds)
+
     @classmethod
     def _missing_(cls, value):
         return None