Update test for Python 3.9.

PiperOrigin-RevId: 433021769

Author: yilei

sonnet/src/base_test.py

@@ -298,7 +298,7 @@ def testEntersNameScope_concreteFunction(self):
 class AbcTest(tf.test.TestCase):
 
   def testAbstract(self):
-    msg = "Can't instantiate .* abstract methods"
+    msg = "Can't instantiate .* abstract method"
     with self.assertRaisesRegex(TypeError, msg):
       AbstractModule()  # pylint: disable=abstract-class-instantiated