docs: def parse_refresh_header(self, refresh):
          """
          >>> parse_refresh_header("1; url=http://example.com/")
          (1.0, 'http://example.com/')
          >>> parse_refresh_header("1; url='http://example.com/'")
          (1.0, 'http://example.com/')
          >>> parse_refresh_header("1")
          (1.0, None)
          >>> parse_refresh_header("blah")  # doctest: +IGNORE_EXCEPTION_DETAIL
          Traceback (most recent call last):
          ValueError: invalid literal for float(): blah
          """
          ii = refresh.find(";")
          if ii != -1:
              pause, newurl_spec = float(refresh[:ii]), refresh[ii+1:]
              jj = newurl_spec.find("=")
              key = None
              if jj != -1:
                  key, newurl = newurl_spec[:jj], newurl_spec[jj+1:]
                  newurl = self.clean_refresh_url(newurl)
              if key is None or key.strip().lower() != "url":
                  raise ValueError()
          else:
              pause, newurl = float(refresh), None
          return pause, newurl