new_contents: import pytest
  
  
  def test_use_twisted():
      pytest.importorskip('twisted')
  
      import txaio
      txaio.use_twisted()
      assert txaio.using_twisted
      assert not txaio.using_asyncio
  
  
  def test_use_twisted_no_twisted():
      # make sure we DO NOT have Twisted installed
      try:
          import twisted  # noqa
          return
      except ImportError:
          pass  # no Twisted
  
      import txaio
      try:
          txaio.use_twisted()
          assert "Should have gotten ImportError"
      except ImportError:
          pass
  
      assert not txaio.using_twisted
      assert txaio.using_asyncio
  
  
  def test_use_asyncio():
      pytest.importorskip('asyncio')
  
      import txaio
      txaio.use_asyncio()
      assert txaio.using_asyncio
      assert not txaio.using_twisted
  
subject: test the use_twisted etc commands