title: Difference between pytz.UTC and pytz.timezone('GMT')
body: <p>pytz's <a href="http://pytz.sourceforge.net/#problems-with-localtime" rel="noreferrer">documentation</a> says:</p>
  
  <blockquote>
    <p>Note that this instance [pytz.timezone('UTC')] is not the same instance (or implementation)
    as other timezones with the same meaning (GMT, Greenwich, Universal,
    etc.).</p>
  </blockquote>
  
  <p>and indeed:</p>
  
  <pre><code>&gt;&gt;&gt; pytz.timezone('UTC') is pytz.timezone('GMT')
  False
  </code></pre>
  
  <p>So... what's the difference?<br>
  When should I use <code>pytz.timezone('UTC')</code> and when should I use <code>pytz.timezone('GMT')</code>?</p>