Python typehints: support type hint decorators with ptransform_fn

  def test_typed_ptransform_fn(self):
    # Test that type hints are propagated to the created PTransform.
    @beam.ptransform_fn
    @typehints.with_input_types(int)
    def MyMap(pcoll):
      return pcoll | beam.ParDo(lambda x: [x])

    self.assertListEqual([1, 2, 3], [1, 2, 3] | MyMap())
    with self.assertRaises(typehints.TypeCheckError):
      _ = ['a'] | MyMap()