title: Getting last modified date of file on FTP server using urlopen not working
body: <pre><code>from six.moves.urllib.request import urlopen, urlretrieve, urlrequest
  
  conn = urlopen('ftp://ftp.cdc.noaa.gov/Datasets/ncep.reanalysis.dailyavgs/surface/lftx.sfc.2017.nc', timeout=20)
  last_modified = conn.headers['last-modified']
  </code></pre>
  
  <p>Following <a href="https://stackoverflow.com/questions/5022083/how-can-i-get-the-last-modified-time-with-python3-urllib">How can I get the last-modified time with python3 urllib?</a> I want to get last modification date of a file on a FTP server. However, with the code above, <code>last-modified</code> is <code>None</code>. Any suggestions? I want solution to work on both python 2 and python 3</p>