[3.12] gh-131045: [Enum] fix flag containment checks when using values (GH-131053) (#131232)

* gh-131045: [Enum] fix flag containment checks when using values (GH-131053)

Check would fail if value would create a pseudo-member, but that member
had not yet been created.  We now attempt to create a pseudo-member for
a passed-in value first.

---------

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>
Co-authored-by: Tomas R. <tomas.roun8@gmail.com>

Lib/enum.py

@@ -771,10 +771,15 @@ class EnumType(type):
         `value` is in `cls` if:
         1) `value` is a member of `cls`, or
         2) `value` is the value of one of the `cls`'s members.
+        3) `value` is a pseudo-member (flags)
         """
         if isinstance(value, cls):
             return True
-        return value in cls._value2member_map_ or value in cls._unhashable_values_
+        try:
+            cls(value)
+            return True
+        except ValueError:
+            return value in cls._unhashable_values_
 
     def __delattr__(cls, attr):
         # nicer error message when someone tries to delete an attribute