* leapseconddata.tai is a timezone object. BEWARE, because of reasons,
it compares equal to datetime.datetime.utc!
* tai_offset can take either kind of timestamp
* to_tai, tai_to_utc: convert to/from the TAI timescale.
* is_leap_second: return True if the given second is a leap second.
In the UTC timescale, the ":59" second returns True even though
"only" the second repetition of :59 is a leap second
