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

Bug report

import datetime
import zoneinfo
from zoneinfo import _zoneinfo

# Requires tzdata==2026.5; use its TZif files rather than system data.
zoneinfo.reset_tzpath([])
utc = datetime.datetime(1976, 12, 1, 1, tzinfo=datetime.UTC)
for implementation in (zoneinfo, _zoneinfo):
    tz = implementation.ZoneInfo.no_cache("Antarctica/Rothera")
    local = utc.astimezone(tz)
    print(implementation.__name__)
    print(local, "fold=", local.fold)
    print(local.astimezone(datetime.UTC))

Output:

zoneinfo
1976-11-30 22:00:00-03:00 fold= 1
1976-12-01 01:00:00+00:00

zoneinfo._zoneinfo
1976-11-30 22:00:00+00:00 fold= 0
1976-11-30 22:00:00+00:00

The pure-Python implementation should give the same result as the C implementation. Rothera's offset moved from UTC to UTC−03 at 1976-12-01 00:00 UTC, so this instant is in the second occurrence of 22:00 local time and should have fold=1. Instead, the result has fold=0 and converting it back to UTC loses three hours.

In the single-transition branch of fromutc(), tti_prev is set to self._ttinfos[-1], which is the offset after the transition. It matches self._tz_after, so the calculated shift is zero. I think this branch needs to use self._tti_before as the previous offset, as the C implementation already does.

Linked PRs

• gh-159706: Fix pure-Python ZoneInfo.fromutc() fold detection for single-transition zones #159828