`Timepoint._get_dump_format()` broken

[MetRonnie]

Instead of CCYY, it uses the year itself

>>> timepoint = TimePoint(year=2010, month_of_year=3, hour_of_day=10, time_zone_hour=-3)
>>> timepoint._get_dump_format()
'2010-MM-DDThh:mm:ss+hh:mm'

This might not be currently affecting anything if _get_dump_format() is only used for dumping/stringifying TimePoints, as the year would replace CCYY anyway.