Update type annotation in get_finite_datetime_ranges_from_timestamps

[con-cat]

Before this change, the finite_datetime_range argument to this function had the type FiniteDatetimeRange.

The function doesn't use any of the methods from that class and works when called on objects of its parent type
FiniteRange[datetime.datetime], so this change updates the type annotation to reflect that.

This caused an issue for me when iterating through a FiniteRangeSet and calling this function - the following works, but mypy currently doesn't allow it:

range_set = FiniteRangeSet(
    [FiniteDatetimeRange(datetime.datetime(2023, 1, 1), datetime.datetime(2023, 1, 31))]
)
for datetime_range in range_set:
    get_finite_datetime_ranges_from_timestamps(datetime_range, [])

Gives this error:

error: Argument 1 to "get_finite_datetime_ranges_from_timestamps" has incompatible type "FiniteRange[datetime]"; expected "FiniteDatetimeRange"  [arg-type]