Cron expression with '?' in Day-of-month and Day-of-Week throws CroniterNotAlphaError exception

[kavilakshmi]

`from datetime import datetime
from croniter import croniter

it = croniter("0/3 * * * ? *", datetime(2021, 1, 1))
for i in range(0, 15):
print(it.get_next(datetime).isoformat())
`

Result :
Traceback (most recent call last):
File "/home/ubuntu/Desktop/oneQuext/quext-scheduler-engine/venv/lib/python3.10/site-packages/croniter/croniter.py", line 190, in _alphaconv
return cls.ALPHACONV[index][key]
KeyError: '?'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/ubuntu/Desktop/oneQuext/quext-scheduler-sync-engine/test_new_scheduler.py", line 74, in
it = croniter("0/3 * * * ? *", datetime(2021, 1, 1)-timedelta(days=1))
File "/home/ubuntu/Desktop/oneQuext/quext-scheduler-engine/venv/lib/python3.10/site-packages/croniter/croniter.py", line 183, in init
self.expanded, self.nth_weekday_of_month = self.expand(expr_format, hash_id=hash_id)
File "/home/ubuntu/Desktop/oneQuext/quext-scheduler-engine/venv/lib/python3.10/site-packages/croniter/croniter.py", line 807, in expand
return cls._expand(expr_format, hash_id=hash_id)
File "/home/ubuntu/Desktop/oneQuext/quext-scheduler-engine/venv/lib/python3.10/site-packages/croniter/croniter.py", line 749, in _expand
t = cls._alphaconv(i, t, expressions)
File "/home/ubuntu/Desktop/oneQuext/quext-scheduler-engine/venv/lib/python3.10/site-packages/croniter/croniter.py", line 192, in _alphaconv
raise CroniterNotAlphaError(
croniter.croniter.CroniterNotAlphaError: [0/3 * * * ? *] is not acceptable

[istrupin]

I'm running into this too -- is this based on a different cron spec perhaps? Would you be open to supporting the ? wildcard given a PR? The wikipedia article linked int he docs does show the ?.

[py-mu]

also，"0 0 * * * ? *" too, Whether there are other solutions?