Display seconds in GMT offsets if available - #1461

I noticed this while working on the skeleton matching.

Per the spec, the localized GMT format includes seconds when applicable, which we currently ignore:

The long format always uses 2-digit hours field and minutes field, with optional 2-digit seconds field. The short format is intended for the shortest representation and uses hour fields without leading zero, with optional 2-digit minutes and seconds fields.

There are real (albeit historic) timezones with sub-minute offsets, e.g. Africa/Monrovia before 1972:

>>> from datetime import datetime
>>> from zoneinfo import ZoneInfo
>>> dt = datetime(1970, 1, 1)
>>> ZoneInfo('Africa/Monrovia').utcoffset(dt)
datetime.timedelta(days=-1, seconds=83730)

I suggest we make get_timezone_gmt display seconds when applicable