title: Seeking from end of file throwing unsupported exception
body: <p>I have this code snippet and I'm trying to seek backwards from the end of file using python:</p>
  
  <pre><code>f=open('D:\SGStat.txt','a');
      f.seek(0,2)
      f.seek(-3,2)
  </code></pre>
  
  <p>This throws the following exception while running:</p>
  
  <pre><code>f.seek(-3,2)
  io.UnsupportedOperation: can't do nonzero end-relative seeks
  </code></pre>
  
  <p>Am i missing something here? </p>