Update croniter to 6.0.0

[srittau]

Make class croniter generic over ret_type and tighten return types for some methods.

Closes: #13260

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

paasta (https://github.com/yelp/paasta)
+ paasta_tools/cli/cmds/validate.py:874: error: List comprehension has incompatible type List[float]; expected List[str]  [misc]

[srittau]

Primer hit is a true positive. The error had been hidden before, since get_next() yielded Any. (But that's only a problem in the annotations. The code only uses the return value for printing.)