stats.sharpe raise error when input is pd.Series.

[g199209]

commit 8ce8784 introduce this bug.

This patch adds a lot of data = data.tz_localize(None) in utils.py, for example in _prepare_prices.

But if input data is pd.Series, not Dataframe, data.tz_localize is wrong.

>>> data = pd.Series([0.000000, 0.000008])
>>> data
0    0.000000
1    0.000008
dtype: float64
>>> data.tz_localize(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mingfei/.local/lib/python3.12/site-packages/pandas/core/generic.py", line 11725, in tz_localize
    ax = _tz_localize(ax, tz, ambiguous, nonexistent)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/mingfei/.local/lib/python3.12/site-packages/pandas/core/generic.py", line 11708, in _tz_localize
    raise TypeError(
TypeError: index is not a valid DatetimeIndex or PeriodIndex