CLN: replace pandas.compat.scipy.scoreatpercentile with numpy.percentile

PR to fix #5824.

- Replaces `compat.scipy.scoreatpercentile` with `numpy.percentile`.
- Sets `axis=0` in a few tests, because `axis=None` by default on `numpy.percentile`, which returns a scalar (i.e., operates on a flattened version of the array).
- Fixes a test that fails after the switch due to differences in how fractions are computed. (Created a `np.timedelta64` directly since `to_timedelta` doesn't support enough precision.)

Let me know if you'd like to see any changes..