gh-159706: Fix pure-Python ZoneInfo.fromutc() fold detection for single-transition zones

[lazerg]

In the pure-Python fromutc(), a zone with exactly one transition takes the timestamp > self._trans_utc[-1] branch, which used self._ttinfos[-1] (the offset after the transition) as tti_prev. That made the shift zero, so times in the fold got fold=0. This uses self._tti_before like the C implementation does, and adds a test.

• Issue: Pure-Python ZoneInfo.fromutc() misses folds in zones with a single transition #159706