gh-100039: enhance __signature__ to work with str and callables (GH-100168)

Callables should be either class- or static-methods.
Enum now uses the classmethod version to greatly improve the help
given for enums and flags.

Lib/enum.py

@@ -685,7 +685,7 @@ class EnumType(type):
                         'member order does not match _order_:\n  %r\n  %r'
                         % (enum_class._member_names_, _order_)
                         )
-
+        #
         return enum_class
 
     def __bool__(cls):
@@ -1083,6 +1083,13 @@ class Enum(metaclass=EnumType):
     attributes -- see the documentation for details.
     """
 
+    @classmethod
+    def __signature__(cls):
+        if cls._member_names_:
+            return '(*values)'
+        else:
+            return '(new_class_name, /, names, *, module=None, qualname=None, type=None, start=1, boundary=None)'
+
     def __new__(cls, value):
         # all enum instances are actually created during class construction
         # without calling this method; this method is called by the metaclass'