Supports TS 3.1

[bowenni]

After this change, the generated Python script is:

"""HelloWorld proxy."""

from google.colab import html

class HelloWorld(html.Element):
  """undefined method."""

  def __init__(self):
    super(HelloWorld, self).__init__('hello-world')

  def say_hi(self, name):
    """sayHi method."""
    self._call('sayHi', name)