fix: accept day-of-week 7 (Sunday) in the seconds and year cron forms

[nikolauspschuetz]

7 is a standard alias for Sunday (0), and croniter accepts it in the classic 5-field form. value_alias skipped the 7 -> 0 conversion in the 6-field (seconds) and 7-field (year) forms, so 7 was rejected there as out of range:

>>> croniter.is_valid("0 0 * * 7")      # 5-field
True
>>> croniter.is_valid("0 0 * * 7 0")    # 6-field
False
>>> croniter.is_valid("0 0 * * 7 0 *")  # 7-field
False

The guard is there to keep rejecting 0 as a day/month in those forms; day-of-week was swept into it. Dropping day-of-week from the two branches makes 7 -> Sunday work in every form while leaving the month/day 0 rejection unchanged. Regression test added; full suite green.