use npy_datetimestruct instead of pandas_datetimestruct
Follow-on to #21863 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21886?src=pr&el=h1) Report
> Merging [#21886](https://codecov.io/gh/pandas-dev/pandas/pull/21886?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bdb61688aa80a6ec9bdb6832946dd83ab4b4c9c8?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21886/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21886?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21886   +/-   ##
=======================================
  Coverage   91.91%   91.91%           
=======================================
  Files         164      164           
  Lines       49992    49992           
=======================================
  Hits        45952    45952           
  Misses       4040     4040
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `42.16% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21886?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21886?src=pr&el=footer). Last update [bdb6168...31e756d](https://codecov.io/gh/pandas-dev/pandas/pull/21886?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @WillAyd thoughts?  Should be easy follow-up to PANDAS_DATETIMEUNIT change thanks @jbrockmendel  when i rebuilt locally after merging this I got an error about finding pandas_datetimeunit, so I think we are missing a dep somewhere (on a np.datetime.pxd?)

works after a ``make clean``