title: Disable Black formatting of dict expression within mapping comprehensions
body: <p>I'm currently researching <a href="https://github.com/psf/black/" rel="noreferrer">Black</a> as our default formatter, but, I'm having some edge cases that don't format well and I want to know if there's a way to get the result I want.</p>
  
  <p><a href="https://github.com/psf/black/" rel="noreferrer">Black</a>'s documentation <a href="https://github.com/psf/black/#the-magic-trailing-comma" rel="noreferrer">partially explores my problem</a>, I have a dictionary expression spread horizontally, and I want to keep it that way since I'm expecting lines to be added, e.g.:</p>
  
  <pre class="lang-py prettyprint-override"><code># Black would keep this as-is because of the trailing comma
  TRANSLATIONS = {
      "en_us": "English (US)",
      "pl_pl": "polski",
  }
  </code></pre>
  
  <p>But in my case the dictionary is inside a list comprehension:</p>
  
  <pre class="lang-py prettyprint-override"><code>res = [
      {
          'id': item.id,
          'name': item.name,
      }
      for item in items.select()
  ]
  </code></pre>
  
  <p>Which Black collapses, regardless of the trailing comma, like so:</p>
  
  <pre class="lang-py prettyprint-override"><code>res = [
      {"id": item.id, "name": item.name,}
      for item in items.select()
  ]
  </code></pre>
  
  <p>Is there a way of telling Black to retain the horizontal structure in these cases?</p>