old_contents: 
  import py
  failure_demo = py.path.local(__file__).dirpath('failure_demo.py')
  pytest_plugins = 'pytester',
  
  def test_failure_demo_fails_properly(testdir):
      target = testdir.tmpdir.join(failure_demo.basename)
      failure_demo.copy(target)
      failure_demo.copy(testdir.tmpdir.join(failure_demo.basename))
      result = testdir.runpytest(target)
      result.stdout.fnmatch_lines([
          "*39 failed*"
      ])
      assert result.ret != 0
  
new_contents: 
  import py
  failure_demo = py.path.local(__file__).dirpath('failure_demo.py')
  pytest_plugins = 'pytester',
  
  def test_failure_demo_fails_properly(testdir):
      target = testdir.tmpdir.join(failure_demo.basename)
      failure_demo.copy(target)
      failure_demo.copy(testdir.tmpdir.join(failure_demo.basename))
      result = testdir.runpytest(target)
      result.stdout.fnmatch_lines([
          "*42 failed*"
      ])
      assert result.ret != 0
  
subject: Fix doctest tox run