Add tasktiger[cron] extra for croniter and pytz

[aaroncoville]

Summary

• setup.py: add extras_require={"cron": ["croniter>=2.0", "pytz>=2024.1"]}.
• cron_expr: replace bare ModuleNotFoundError with clear ImportError pointing to pip install tasktiger[cron].
• README.rst: document tasktiger[cron] install path in cron_expr section.
• tests/test_schedule_cron.py: happy path + both missing-dep paths.
• Rolled into 0.25.1 CHANGELOG entry — minor merge needed.

Repro (before)

pip install tasktiger==0.25.0
python -c "from tasktiger.schedule import cron_expr; cron_expr('0 * * * *')"
# → ModuleNotFoundError: No module named 'croniter'

Verification (after)

pip install '.[cron]'
python -c "
import datetime; from tasktiger.schedule import cron_expr
fn, args = cron_expr('0 * * * *')
print(fn(datetime.datetime(2026, 1, 1, 0, 30), *args))
"
# → 2026-01-01 01:00:00

Base install now gives:

ImportError: tasktiger.schedule.cron_expr requires 'croniter' and 'pytz' (missing: 'croniter'). Install: pip install tasktiger[cron]