chore: Address pandas deprecation warning in test

[MarcoGorelli]

pandas is becoming more case-sensitive about these aliases, to avoid issues like this:

In [3]: pd.date_range('2000', periods=3, freq='NS')  # nanosecond
  pd.date_range('2000', periods=3, freq='NS')
Out[3]:
DatetimeIndex([          '2000-01-01 00:00:00',
               '2000-01-01 00:00:00.000000001',
               '2000-01-01 00:00:00.000000002'],
              dtype='datetime64[ns]', freq='ns')

In [4]: pd.date_range('2000', periods=3, freq='US')  # microsecond
  pd.date_range('2000', periods=3, freq='US')
Out[4]:
DatetimeIndex([       '2000-01-01 00:00:00', '2000-01-01 00:00:00.000001',
               '2000-01-01 00:00:00.000002'],
              dtype='datetime64[ns]', freq='us')

In [5]: pd.date_range('2000', periods=3, freq='MS')  # wait, it's not millisecond?
Out[5]: DatetimeIndex(['2000-01-01', '2000-02-01', '2000-03-01'], dtype='datetime64[ns]', freq='MS')

'h' works across versions without warnings

[emilykl]

H has been deprecated since pandas version 2.2.0.

Here is the documentation page with the deprecation notice: https://pandas.pydata.org/docs/user_guide/timeseries.html#timeseries-offset-aliases

Thank you @MarcoGorelli for this change!