maint: pandas can now index with np.timedelta64

[mathause]

pandas-dev/pandas#20393 was resolved in pandas version 0.23, so this should no longer be necessary. The relevant test is

xarray/xarray/tests/test_dataarray.py

Lines 955 to 957 in 063606b

	array = DataArray(np.arange(365), [("delta", times - times[0])])
	result = array.sel(delta=slice(array.delta[0], array.delta[-1]))
	assert_equal(result, array)

so let's see if this passes

[max-sixty]

Thanks @mathause