Add tasktiger[cron] extra for croniter and pytz

[aaroncoville]

Summary

• Added extras_require={"cron": ["croniter>=2.0", "pytz>=2024.1"]} to setup.py.
• tasktiger.schedule.cron_expr now raises a clear ImportError pointing users at pip install tasktiger[cron] when either dependency is missing (previously a basic ModuleNotFoundError).
• Documented the tasktiger[cron] install path in the cron_expr section of README.rst.
• Added tests/test_schedule_cron.py covering the happy path and both missing-dependency paths.
• Rolled up under the 0.25.1 CHANGELOG.md entry from my earlier PR.
  • Will require a minor merge of this document

Reproduction (before the fix)

python -m venv /tmp/tt-base
/tmp/tt-base/bin/pip install tasktiger==0.25.0
/tmp/tt-base/bin/python -c "
import datetime
from tasktiger.schedule import cron_expr
fn, args = cron_expr('0 * * * *')
fn(datetime.datetime(2026, 1, 1, 0, 30), *args)
"
# → ModuleNotFoundError: No module named 'croniter'

Verification (after the fix)

Extras install works end-to-end:

python -m venv /tmp/tt-extra && /tmp/tt-extra/bin/pip install '.[cron]'
/tmp/tt-extra/bin/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 produces a clear error:

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