Fix doctest tox run

The new failures in the failure demo apparently affect the
doctests tox run.

doc/en/example/assertion/test_failures.py

@@ -9,6 +9,6 @@ def test_failure_demo_fails_properly(testdir):
     failure_demo.copy(testdir.tmpdir.join(failure_demo.basename))
     result = testdir.runpytest(target)
     result.stdout.fnmatch_lines([
-        "*39 failed*"
+        "*42 failed*"
     ])
     assert result.ret != 0