[3.9] bpo-41789: honor object overrides in Enum classes (GH-22250) (GH-22272)

EnumMeta double-checks that `__repr__`, `__str__`, `__format__`, and `__reduce_ex__` are not the same as `object`'s, and replaces them if they are -- even if that replacement was intentionally done in the Enum being constructed.  This patch fixes that.
diff --git a/Lib/enum.py b/Lib/enum.py
index 9dfba3c..8711558 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -249,7 +249,11 @@
 
         # double check that repr and friends are not the mixin's or various
         # things break (such as pickle)
+        # however, if the method is defined in the Enum itself, don't replace
+        # it
         for name in ('__repr__', '__str__', '__format__', '__reduce_ex__'):
+            if name in classdict:
+                continue
             class_method = getattr(enum_class, name)
             obj_method = getattr(member_type, name, None)
             enum_method = getattr(first_enum, name, None)