BUG: astype of datetimelike to non-NS
The current pandas master includes a behaviour change date conversion.

Before #19176 

```
In [1]: import pandas as pd

In [2]: pd.DataFrame({'x': [3, 2, 1]}).astype('M8[us]')
Out[2]:
                           x
0 1970-01-01 00:00:00.000003
1 1970-01-01 00:00:00.000002
2 1970-01-01 00:00:00.000001
```

After #19176 

```
In [1]: import pandas as pd

In [2]: pd.DataFrame({'x': [3, 2, 1]}).astype('M8[us]')
Out[2]:
                              x
0 1970-01-01 00:00:00.000000003
1 1970-01-01 00:00:00.000000002
2 1970-01-01 00:00:00.000000001
```