onstructor.

        is_dst is used to determine the correct timezone in the ambigous
        period at the end of daylight saving time.

        >>> from pytz import timezone
        >>> fmt = '%Y-%m-%d %H:%M:%S %Z (%z)'
        >>> amdam = timezone('Europe/Amsterdam')
        >>> dt  = datetime(2004, 10, 31, 2, 0, 0)
        >>> loc_dt1 = amdam.localize(dt, is_dst=True)
        >>> loc_dt2 = amdam.localize(dt, is_dst=False)
        >>> loc_dt1.strftime(fmt)
        '2004-10-31 02:00:00 CEST (+0200)'
        >>> loc_dt2.strftime(fmt)
        '2004-10-31 02:00:00 CET (+0100)'
        >>> str(loc_dt2 - loc_dt1)
        '1:00:00'

        Use is_dst=None to raise an AmbiguousTimeError for ambiguous
        times at the end of daylight saving time

        >>> try:
        ...     loc_dt1 = amdam.localize(dt, is_dst=None)
        ... except AmbiguousTimeError:
        ...     print('Ambiguous')
        Ambiguous

        is_dst defaults to False

        >>> amdam.localize(dt) == amdam.localize(dt, False)
        True

        is_dst is also used to determine the correct timezone in the
        wallclock times jumped over at the start of daylight saving time.

        >>> pacific = timezone('US/Pacific')
        >>> dt = datetime(2008, 3, 9, 2, 0, 0)
        >>> ploc_dt1 = pacific.localize(dt, is_dst=True)
        >>> ploc_dt2 = pacific.localize(dt, is_dst=False)
        >>> ploc_dt1.strftime(fmt)
        '2008-03-09 02:00:00 PDT (-0700)'
        >>> ploc_dt2.strftime(fmt)
        '2008-03-09 02:00:00 PST (-0800)'
        >>> str(ploc_dt2 - ploc_dt1)
        '1:00:00'

        Use is_dst=None to raise a NonExistentTimeError for these skipped
        times.

        >>> try:
        ...     loc_dt1 = pacific.localize(dt, is_dst=None)
        ... except NonExistentTimeError:
        ...     print('Non-existent')
        Non-existent
        Nr7