Fix date_range(start, end, freq) when end-start is divisible by freq

[mroeschke]

Description

xref #16507

date_range generates its dates via range, and the end of this range was calculated via math.ceil((end - start) / freq). If (end - start) / freq did not produce a remainder, math.ceil would not correctly increment this value by 1 to capture the last date.

Instead, this PR uses math.floor((end - start) / freq) + 1 to always ensure the last date is captured

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

Do we need to add some test cases for non-integer multiples too? Also for reference I think this is how pandas calculates the space: https://github.com/pandas-dev/pandas/blob/93198fb47a754051f84c235dea8920ffc3cf1b13/pandas/core/arrays/datetimes.py#L511-L514

[mroeschke]

Do we need to add some test cases for non-integer multiples too?

IIUC, there is a test python/cudf/cudf/tests/test_datetime.py::test_date_range_start_end_freq that tests resulting periods that are non-integer

[mroeschke]

/merge