`np.timedelta64`: update to track deprecation of `generic` unit in NumPy

[d-v-b]

Looks like the "generic" unit is being removed from timedelta64 in NumPy, and there are discussions about using a concrete default unit:

• BUG: timedelta64 comparison is not transitive numpy/numpy#28287
• DEP: Deprecate 'generic' unit in np.timedelta64 numpy/numpy#29619

When this is merged, we will need to update the data type spec for np.timedelta64 accordingly.

[d-v-b]

this is tedious, it would be great to normalize a better data type for dates / times that doesn't need an update when numpy changes its behavior