title: pytest raise PytestUnknownMarkWarning for pytest.mark.optionalhook in conftest
body: <p>In pytest files, I have a main.py for test entrance and have such code in conftest.py:</p>
  <pre><code>@pytest.mark.optionalhook
  def pytest_html_results_table_row(report, cells):
      # my code
  </code></pre>
  <p>and in another py file, I use os.system to call the main.py above</p>
  <pre><code>os.system('python main.py')
  </code></pre>
  <p>but there's warning message like:</p>
  <pre><code>PytestUnknownMarkWarning: Unknown pytest.mark.optionalhook
  </code></pre>
  <p>I'm curious, <code>pytest.mark.optionalhook</code> is provided by pytest structure. Why is there still warning?</p>