title: emoji from output is different on Windows and Linux
body: <p>On Windows:</p>
  <pre class="lang-py prettyprint-override"><code>import re
  import subprocess
  
  output = subprocess.run(['python', '-m', 'black'], stderr=subprocess.PIPE)
  stderr = output.stderr.decode()
  assert re.match(r'No Path provided. Nothing to do \\U0001f634\r\n', stderr)
  </code></pre>
  <p>On Linux, however, I need to change the regular expression in two places:</p>
  <pre class="lang-py prettyprint-override"><code>import re
  import subprocess
  
  output = subprocess.run(['python', '-m', 'black'], stderr=subprocess.PIPE)
  stderr = output.stderr.decode()
  assert re.match(r'No Path provided. Nothing to do \U0001f634\n', stderr)
  </code></pre>
  <p>I understand why I need to change <code>\r\n</code> to <code>\n</code> - it's because of the <a href="https://en.wikipedia.org/wiki/Carriage_return" rel="nofollow noreferrer">carriage return</a></p>
  <p>However, why do I need to change <code>'\\U0001f634'</code> to <code>'\U0001f634'</code>? Is there a way of writing the above assertion in a cross-platform way?</p>