code: def parse_line(self, line: str) -> None:
          
          if line[0].isspace():
              # continuation of a multi-line header
              if self._last_key is None:
                  raise HTTPInputError("first header line cannot start with whitespace")
              new_part = " " + line.lstrip()
              self._as_list[self._last_key][-1] += new_part
              self._dict[self._last_key] += new_part
          else:
              try:
                  name, value = line.split(":", 1)
              except ValueError:
                  raise HTTPInputError("no colon in header line")
              self.add(name, value.strip())
docs: Updates the dictionary with a single header line.
  
          >>> h = HTTPHeaders()
          >>> h.parse_line("Content-Type: text/html")
          >>> h.get('content-type')
          'text/html'