Message 134830 - Python tracker

Since this has been reported already several times (see e.g. #11947), and it's a fairly common mistake, I think we should do something to avoid it.

A few possibilities are:
  1) add a warning in the doc;
  2) make count and flag keyword-only argument (raising a deprecation warning in 3.3 and actually change it later);
  3) change the regex flags to some object that can be distinguished from ints and raise an error when a flag is passed to count;