API: deprecate silently casting np.nan to NaT
When doing a setitem into a dt64/td64 Series, we cast `np.nan` to `NaT`.  The alternative would be to cast to object dtype and set np.nan unchanged.

Similarly anything that ends up calling `is_valid_nat_for_dtype` will allow through np.nan which ends up getting cast to NaT.

We should consider deprecating this behavior and requiring users to pass NaT directly.