Timedelta/TimedeltaIndex does not accept "unit" higher than ns

[jbrockmendel]

>>> td = np.timedelta64(1000, 'ps')
>>> pd.Timedelta(td).value
1
>>> pd.Timedelta(1000, unit='ps')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/_libs/tslibs/timedeltas.pyx", line 1020, in pandas._libs.tslibs.timedeltas.Timedelta.__new__
  File "pandas/_libs/tslibs/timedeltas.pyx", line 136, in pandas._libs.tslibs.timedeltas.convert_to_timedelta64
  File "pandas/_libs/tslibs/timedeltas.pyx", line 225, in pandas._libs.tslibs.timedeltas.cast_from_unit
ValueError: cannot cast unit ps

[gfyoung]

I wasn't ever aware that we could use more fine-grained units that ns in the first place in pandas, though I don't see why we couldn't support it...