fix: make upstream tests pass

[d-v-b]

We have 2 failures in our upstream tests. One is caused by NumPy deprecating the creation of a NaT datetime without an explicitly declared unit, and the fix is to declare a unit when creating a NaT datetime. The second failure has an unknown cause but was fixed by bumping the version of ml_dtypes used in an example script.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.11%. Comparing base (8217046) to head (e41a98f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3920   +/-   ##
=======================================
  Coverage   93.11%   93.11%           
=======================================
  Files          85       85           
  Lines       11365    11365           
=======================================
  Hits        10582    10582           
  Misses        783      783           

Files with missing lines	Coverage Δ
src/zarr/core/dtype/npy/time.py	98.85% <100.00%> (ø)

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