message: Support Python 3.9 in testIsAbstract
diff: @@ -36,9 +36,10 @@ class BaseBijectorTest(test.TestCase):
  """Tests properties of the Bijector base-class."""
  def testIsAbstract(self):
  + # In Python 3.9, "abstract methods" become "abstract method"
  with self.assertRaisesRegex(TypeError,
  ("Can't instantiate abstract class Bijector "
  - "with abstract methods __init__")):
  + "with abstract methods? __init__")):
  bijector.Bijector() # pylint: disable=abstract-class-instantiated
  def testDefaults(self):
  