title: Flake8: Ignore specific warning for entire file
body: <p>The <a href="http://flake8.pycqa.org/en/3.1.1/user/ignoring-errors.html" rel="noreferrer"><em>Ignoring Errors</em></a> docs currently list a way of ignoring a particular error for a particular line:</p>
  
  <pre><code>example = lambda: 'example'  # noqa: E731
  </code></pre>
  
  <p>... and a way of ignoring <em>all</em> errors for an entire file:</p>
  
  <pre><code># flake8: noqa
  
  from foo import unused
  function_that_doesnt_exist()
  x = 1+       2
  </code></pre>
  
  <p>... and a couple of ways, either through config or through command-line options, of disabling a particular error <em>globally</em> across an entire project.</p>
  
  <p>But what if I want to ignore a <em>particular</em> error across the entirety of a <em>single</em> file - for instance, to disable warnings about unused imports in an <code>__init__.py</code> barrel file that just imports a bunch of classes so that code from other packages can import them from it in turn? The docs don't seem to hint at any syntax for this. Is it possible?</p>