bpo-42901: [Enum] move member creation to __set_name__ (GH-24196)

`type.__new__` calls `__set_name__` and `__init_subclass__`, which means
that any work metaclasses do after calling `super().__new__()` will not
be available to those two methods.  In particular, `Enum` classes that
want to make use of `__init_subclass__` will not see any members.

Almost all customization is therefore moved to before the
`type.__new__()` call, including changing all members to a proto member
descriptor with a `__set_name__` that will do the final conversion of a
member to be an instance of the `Enum` class.

Lib/inspect.py

@@ -408,7 +408,7 @@ def classify_class_attrs(cls):
     # attribute with the same name as a DynamicClassAttribute exists.
     for base in mro:
         for k, v in base.__dict__.items():
-            if isinstance(v, types.DynamicClassAttribute):
+            if isinstance(v, types.DynamicClassAttribute) and v.fget is not None:
                 names.append(k)
     result = []
     processed = set()