instead of int + negative_utc boolean
The plan:
1. Add `offset_bytes` attribute to `TimestampWithTimezone`, fill it from `offset` and `negative_utc` if not provided
2. Make `offset` and `negative_utc` optional (fill the attributes from `offset_str`)
3. Raise DeprecationWarning when they are passed to the constructor (but `from_dict` should not raise it)
4. Remove explicit uses in the constructor (track down deprecation warnings)
5. Remove use of `negative_utc` (there is one in `swh-web/swh/web/common/converters.py`)
6. Remove them from the constructor (but not `from_dict`, because kafka), make `offset` a property (can be useful to display), remove `negative_utc` entirely (useless unless we need to reserialize)
Postgresql must be migrated somewhere between 1 and 6 (exclusive). Kafka does not need to be migrated yet.