Unreachable case in internals.make_block?
https://github.com/pandas-dev/pandas/blob/master/pandas/core/internals.py#L2937

```
        elif issubclass(vtype, np.datetime64):
            if hasattr(values, 'tz'):
                klass = DatetimeTZBlock
            else:
                klass = DatetimeBlock
```

The `hasattr(values, 'tz')` condition is never True in tests.  I'm not aware of any scenario in which it could be.  Am I missing something?