title: Extended interpolation not working in configparser
body: <p>I tried to use configparser module from standard library in python 3.6 or python 3.5.1</p>
  
  <p>My ini file looks like this:</p>
  
  <pre class="lang-none prettyprint-override"><code>[common]
  domain = http://some_domain_name:8888
  about = about/
  loginPath = /accounts/login/?next=/home
  fileBrowserLink = /filebrowser
  partNewDirName = some_dir
  
  [HUE_310]
  partNewFilePath = ${common:domain}
  </code></pre>
  
  <p>My "main" program looks like this:</p>
  
  <pre><code>from configparser import ConfigParser
  
  parser = ConfigParser()
  parser.read('configfile.ini')
  
  lll = parser.get('HUE_310', 'partNewFilePath')
  print(lll)
  </code></pre>
  
  <p>Instead <code>http://some_domain_name:8888</code> I got <code>${common:domain}</code></p>
  
  <p>I use Pycharm Community as my IDE. I use virtualenv.</p>
  
  <p>I have no idea what is wrong with my code...</p>