title: Pytest-pep8 problem while following Head First Python 2nd edition
body: <p>I've been following Head First Python 2nd Edition and in chapter 4, a pep8 compliance testing is demonstrated. Despite having the same code as in book, my output is different.</p>
  
  <p>Tested code is quite simple:</p>
  
  <pre class="lang-py prettyprint-override"><code>def search4vowels(phrase: str) -&gt; set:
      """Return vowels found in supplied phrase."""
      vowels = set('aeiou')
      return vowels.intersection(set(phrase))
  
  
  def search4letters(phrase: str, letters: str='aeiou') -&gt; set:
      """Return a set of 'letters' found in 'phrase'."""
      return set(letters).intersection(set(phrase))
  </code></pre>
  
  <p>I tried testing as shown in book, and got this output:</p>
  
  <pre><code>λ py.test.exe --pep8 vsearch.py
  c:\users\gx\appdata\local\programs\python\python37-32\lib\site-packages\pep8.py:110: FutureWarning: Possible nested set at position 1
    EXTRANEOUS_WHITESPACE_REGEX = re.compile(r'[[({] | []}),;:]')
  ============================= test session starts =============================
  platform win32 -- Python 3.7.4, pytest-5.1.1, py-1.8.0, pluggy-0.12.0
  rootdir: C:\Users\gx\Desktop\H.F. Python\mymodules
  plugins: pep8-1.0.6
  collected 1 item
  
  vsearch.py .                                                             [100%]
  
  ============================== warnings summary ===============================
  c:\users\gx\appdata\local\programs\python\python37-32\lib\site-packages\_pytest\mark\structures.py:324
    c:\users\gx\appdata\local\programs\python\python37-32\lib\site-packages\_pytest\mark\structures.py:324:
  PytestUnknownMarkWarning: Unknown pytest.mark.pep8 - is this a typo?
  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
      PytestUnknownMarkWarning,
  
  -- Docs: https://docs.pytest.org/en/latest/warnings.html
  ======================== 1 passed, 1 warnings in 0.04s ========================
  </code></pre>
  
  <p>(I slightly modified whitespaces to make is more readable)</p>
  
  <p>In the book no such thing is occurs.
  Before doing this I have freshly installed pytest and pytest-pep8 as shown in book.</p>
  
  <p>What causes this?</p>