bpo-43475: Fix the Python implementation of hash of Decimal NaN (GH-26679)

(cherry picked from commit 9f1c5f6e8a)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/test/test_decimal.py

@@ -1814,13 +1814,7 @@ def hashit(d):
 
         # check that hash(d) == hash(int(d)) for integral values
         for value in test_values:
-            self.assertEqual(hashit(value), hashit(int(value)))
-
-        #the same hash that to an int
-        self.assertEqual(hashit(Decimal(23)), hashit(23))
-        self.assertRaises(TypeError, hash, Decimal('sNaN'))
-        self.assertTrue(hashit(Decimal('Inf')))
-        self.assertTrue(hashit(Decimal('-Inf')))
+            self.assertEqual(hashit(value), hash(int(value)))
 
         # check that the hashes of a Decimal float match when they
         # represent exactly the same values
@@ -1829,7 +1823,7 @@ def hashit(d):
         for s in test_strings:
             f = float(s)
             d = Decimal(s)
-            self.assertEqual(hashit(f), hashit(d))
+            self.assertEqual(hashit(d), hash(f))
 
         with localcontext() as c:
             # check that the value of the hash doesn't depend on the
@@ -1850,6 +1844,19 @@ def hashit(d):
             x = 1100 ** 1248
             self.assertEqual(hashit(Decimal(x)), hashit(x))
 
+    def test_hash_method_nan(self):
+        Decimal = self.decimal.Decimal
+        self.assertRaises(TypeError, hash, Decimal('sNaN'))
+        value = Decimal('NaN')
+        self.assertEqual(hash(value), object.__hash__(value))
+        class H:
+            def __hash__(self):
+                return 42
+        class D(Decimal, H):
+            pass
+        value = D('NaN')
+        self.assertEqual(hash(value), object.__hash__(value))
+
     def test_min_and_max_methods(self):
         Decimal = self.decimal.Decimal