BUG: DatetimeArray.unit when constructed from a non-nano ndarray
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/v2.1.0.rst` file if fixing a bug or adding a new feature.

fixes the following inconsistency:

```
from pandas.core.arrays.datetimes import DatetimeArray
import numpy as np

data = np.array([1], dtype="datetime64[s]")
dta = DatetimeArray(data)

print(dta.unit)     # ns
print(dta[0].unit)  # s
```
Thanks @lukemanley 