BUG: numpy.datetime64 unit counts ignored
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---

```python
import numpy as np
import pandas as pd
pd.Timestamp(np.datetime64('2000-01-01', '10ns')) == pd.Timestamp('2000-01-01')
```

Should be ```True```, is instead ```False```. The problem is that the unit count of 10 is ignored. Compare:

```python
pd.Timestamp(np.datetime64('2000-01-01', '10ns')).asm8.item()
pd.Timestamp(np.datetime64('2000-01-01', '1ns')).asm8.item()                                        
```

The bug occurs in two places, both in `_libs/tslibs/conversion.pyx`. Both `get_datetime64_nanos` and `ensure_datetime64ns` forget to check the count. I don't know a Cython API to get the count, though it is accessible from `numpy.datetime_data`.