sequence_to_dt64ns doesn't handle non-ndarray sequences with a dtype
For example an `xarray.DataArray`.

```python
In [21]: import xarray as xr

In [22]: import pandas as pd; import numpy as np

In [23]: arr = xr.DataArray(pd.date_range('2000', periods=4).to_numpy())

In [24]: pd.core.arrays.datetimes.sequence_to_dt64ns(arr)
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-24-fd995f4e5bef> in <module>
----> 1 pd.core.arrays.datetimes.sequence_to_dt64ns(arr)

~/sandbox/pandas/pandas/core/arrays/datetimes.py in sequence_to_dt64ns(data, dtype, copy, tz, dayfirst, yearfirst, ambiguous)
   1712         result = result.copy()
   1713
-> 1714     assert isinstance(result, np.ndarray), type(result)
   1715     assert result.dtype == 'M8[ns]', result.dtype
   1716

AssertionError: <class 'xarray.core.dataarray.DataArray'>
```

A user would hit this via `DatetimeIndex(arr)`