Issue #21271: Adds new keyword only parameters in reset_mock call

We now have two keyword only parameters in the reset_mock function to
selectively reset the return_value or the side_effects, or both.
diff --git a/Lib/unittest/test/testmock/testmock.py b/Lib/unittest/test/testmock/testmock.py
index 9910ab9..b07a7cc 100644
--- a/Lib/unittest/test/testmock/testmock.py
+++ b/Lib/unittest/test/testmock/testmock.py
@@ -1277,6 +1277,24 @@
         self.assertEqual(m.method_calls[0], c)
         self.assertEqual(m.method_calls[1], i)
 
+    def test_reset_return_sideeffect(self):
+        m = Mock(return_value=10, side_effect=[2,3])
+        m.reset_mock(return_value=True, side_effect=True)
+        self.assertIsInstance(m.return_value, Mock)
+        self.assertEqual(m.side_effect, None)
+
+    def test_reset_return(self):
+        m = Mock(return_value=10, side_effect=[2,3])
+        m.reset_mock(return_value=True)
+        self.assertIsInstance(m.return_value, Mock)
+        self.assertNotEqual(m.side_effect, None)
+
+    def test_reset_sideeffect(self):
+        m = Mock(return_value=10, side_effect=[2,3])
+        m.reset_mock(side_effect=True)
+        self.assertEqual(m.return_value, 10)
+        self.assertEqual(m.side_effect, None)
+
     def test_mock_add_spec(self):
         class _One(object):
             one = 1