title: Pylint complains about comparing a string to a literal with 'is'
body: <p>Consider this code snippet:</p>
  <pre><code>my_string = 'asdf'
  print(my_string is 'xfje') #R0123
  </code></pre>
  <p>Pylint returns a recommendation <code>R0123</code> on the second line, which I was unable to find on the <a href="http://pylint-messages.wikidot.com" rel="noreferrer">error message wiki</a>. There is a mention of it in <a href="https://pylint.readthedocs.io/en/latest/technical_reference/features.html" rel="noreferrer">this part of the docs</a>, though:</p>
  <blockquote>
  <p><strong>literal-comparison (R0123):</strong></p>
  <p>Comparison to literal Used when comparing an object to a literal, which is usually what you do not want to do, since you can compare to a different literal than what was expected altogether.</p>
  </blockquote>
  <p>This explanation is not helpful at all to me. I know that using <code>is</code> for comparison between two string objects may lead to different results than expected, but for comparison of object to literal, it is identical to <code>==</code> . And when using <code>==</code>, the error disappears.</p>
  <p>Why should I not use <code>is</code> here?</p>