docs: def _parse_attr(self, line):
          """Parse a single attribute type/value pair."""
          colon_pos = line.index(b':')
          attr_type = line[0:colon_pos].decode('ascii')
  
          if line[colon_pos:].startswith(b'::'):
              attr_value = base64.decodestring(line[colon_pos + 2:])
          elif line[colon_pos:].startswith(b':<'):
              url = line[colon_pos + 2:].strip()
              attr_value = b''
              if self._process_url_schemes:
                  u = urlparse(url)
                  if u[0] in self._process_url_schemes:
                      attr_value = urlopen(url.decode('ascii')).read()
          else:
              attr_value = line[colon_pos + 1:].strip()
  
          return self._decode_value(attr_type, attr_value)