API: Index(object_dtype_bool_ndarray) retain object dtype
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Just to solidify the "rule of thumb" where this is headed

```
pd.Index/Series(np.array([...], dtype=object), dtype=None) -> object
pd.Index/Series(np.array([...], dtype=object), dtype=other) == pd.Index/Series(np.array([...], dtype=object)).astype(other)
```

? Thanks @jbrockmendel  > Just to solidify the "rule of thumb" where this is headed [...]

Not quite.  In the `pd.Index/Series(ndarray_object)` case we'll continue to do inference on datetime, timedelta, interval, and period.

(and more generally id have no strong objection to doing inference on numeric/bool dtypes, just as long as we're consistent about it)