bpo-32265: Classify class and static methods of builtin types. (#4776)

Add types.ClassMethodDescriptorType for unbound class methods.
diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index 47488a6..56848c1 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -594,6 +594,10 @@
         self.assertIsInstance(''.join, types.BuiltinMethodType)
         self.assertIsInstance([].append, types.BuiltinMethodType)
 
+        self.assertIsInstance(int.__dict__['from_bytes'], types.ClassMethodDescriptorType)
+        self.assertIsInstance(int.from_bytes, types.BuiltinMethodType)
+        self.assertIsInstance(int.__new__, types.BuiltinMethodType)
+
 
 class MappingProxyTests(unittest.TestCase):
     mappingproxy = types.MappingProxyType