[croniter] croniter.pyi: `get_next` with a `ret_type` passed should return that type

[anentropic]

I think this was maybe a typo?

Currently I am getting spurious errors from Pyright and I think it's due to this mismatch:

[github-actions[bot]]

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:879: error: List comprehension has incompatible type List[float]; expected List[str]  [misc]
+ paasta_tools/cli/cmds/validate.py:879: error: List comprehension has incompatible type List[datetime]; expected List[str]  [misc]

[anentropic]

https://github.com/yelp/paasta

their type annotation seems wrong either way...