Version 5:
 - add "datetime" and "timespec" formats: datetime.datetime object and  (sec: int, nsec: int)
 - add timestamp optional format to os.stat(), os.lstat(), os.fstat(), os.fstatat()
 - support passing the timestamp format as a keyword: time.time(format="decimal")

I am not really conviced by the usefulness of "timespec" format, but it was just an example for #11457.

The "datetime" format is surprising for time.clock() and time.wallclock(), these timestamps use an arbitrary start. I suppose that time.clock(format="datetime") and time.wallclock(format="datetime") should raise a ValueError.