[3.12] gh-130230: Add new tests for 3-arg pow() with Decimal (GH-130237) (GH-130247)

(cherry picked from commit b93b7e5)

Author: serhiy-storchaka

Lib/test/test_decimal.py

@@ -4463,6 +4463,15 @@ def test_implicit_context(self):
             self.assertIs(Decimal("NaN").fma(7, 1).is_nan(), True)
             # three arg power
             self.assertEqual(pow(Decimal(10), 2, 7), 2)
+            if self.decimal == C:
+                self.assertEqual(pow(10, Decimal(2), 7), 2)
+                self.assertEqual(pow(10, 2, Decimal(7)), 2)
+            else:
+                # XXX: Three-arg power doesn't use __rpow__.
+                self.assertRaises(TypeError, pow, 10, Decimal(2), 7)
+                # XXX: There is no special method to dispatch on the
+                # third arg of three-arg power.
+                self.assertRaises(TypeError, pow, 10, 2, Decimal(7))
             # exp
             self.assertEqual(Decimal("1.01").exp(), 3)
             # is_normal