docs: def fetch_url(src, dst):
      """
      Fetch file from URL src and save it to dst.
      """
      # we do not use the nicer sys.version_info.major
      # for compatibility with Python < 2.7
      if sys.version_info[0] > 2:
          import urllib.request
  
          class URLopener(urllib.request.FancyURLopener):
              def http_error_default(self, url, fp, errcode, errmsg, headers):
                  sys.stderr.write("ERROR: could not fetch {0}\n".format(url))
                  sys.exit(-1)
      else:
          import urllib
  
          class URLopener(urllib.FancyURLopener):
              def http_error_default(self, url, fp, errcode, errmsg, headers):
                  sys.stderr.write("ERROR: could not fetch {0}\n".format(url))
                  sys.exit(-1)
  
      dirname = os.path.dirname(dst)
      if dirname != '':
          if not os.path.isdir(dirname):
              os.makedirs(dirname)
  
      opener = URLopener()
      opener.retrieve(src, dst)