timezone": UTC. In fact, when datetime.timezone was introduced in Python, support for UTC was the main motivation. Arbitrary fixed offset timezones were added primarily because once we've already paid a price of adding a timezone class to the datetime module, support for arbitrary fixed offset timezones came essentially for free.
Non-UTC fixed offset timezones are quite useful when your timestamps come with UTC offset information. Once you parse such timestamps into aware datetime instances (using strptime or your own parser), you can do almost everything you want with the result: convert to UTC, reformat/serialize for further transmission, compute time differences, etc. The only operation that is slightly odd is the addition of a timedelta which may give you a result that uses winter time offset in the summer. Note that the result is not wrong - just not what some users may expect.