BUG: Allow DatetimeIndex for scipy interpoalte
Merge pull request #5977 from TomAugspurger/interpolate-datetime

BUG: Allow DatetimeIndex for scipy interpolate
PERF: perf improvments in indexing with object dtypes (GH5968)
BUG: Bug in NaT comparison if a mixed datetime/np.datetime64 with NaT were passed (5968)
CLN: cleanup nat comparisons in tslib.pyx