title: Clock.schedule_interval doesn't schedule callback
body: <p>I am testing the functionality of the kivy.clock.Clock.schedule_interval function. </p>
  
  <p>My schedule_interval isn't calling the test function but rather exits without any errors.</p>
  
  <p>What is it that I'm not understanding? I have correctly modeled this test by the documentation.</p>
  
  <pre><code>from kivy.clock import Clock
  
  class TestClass:
  
      def __init__(self):
          print("function __init__.")
          schedule = Clock.schedule_interval(self.test, 1)
  
      def test(self, dt):
          print("function test.")
  
  if __name__ == '__main__':
      a = TestClass()
  </code></pre>
  
  <p>The expected output should be:</p>
  
  <pre><code>function __init__.
  function test.
  function test.
  function test.
  function test.
  function test.
  function test.
  </code></pre>
  
  <p>Instead I'm just getting:</p>
  
  <pre><code>function __init__.
  </code></pre>