title: Black --exclude argument not excluding desired file(s)
body: <p>Let's say I have the following python file <code>exclude_from_black.py</code> in the root of my project:</p>
  
  <pre><code>print('I want single quotes')
  </code></pre>
  
  <p>I'm trying to exclude this file from black reformatting but none of the following commands seem to work:</p>
  
  <pre><code>black --exclude="exclude_from_black.py"
  black --exclude="exclude_from_black\.py"
  black --exclude="exclude\_from\_black\.py"
  </code></pre>
  
  <p>According to the <a href="https://github.com/psf/black#command-line-options" rel="noreferrer">Documentation</a> the <code>exclude</code> argument takes a regex. What am I doing something wrong here?</p>