Should DatetimeTZDtype normalize the timezone?
The `.tz` on a `Timestamp` can differ from the `.tz` on the array / index it's contained within

```python
In [5]: idx = pd.date_range('2000', periods=4, tz="US/Central")

In [6]: idx.tz
Out[6]: <DstTzInfo 'US/Central' LMT-1 day, 18:09:00 STD>

In [7]: idx[0].tz
Out[7]: <DstTzInfo 'US/Central' CST-1 day, 18:00:00 STD>
```

I gather that this is the correct behavior. My question: since DatetimeTZDtype is only really used when attached to an array, should it automatically normalize the user-provided `tz` to the version that's attached to the array? In code, both of the following

```python
In [11]: pd.DatetimeTZDtype(tz=idx.tz).tz
Out[11]: <DstTzInfo 'US/Central' LMT-1 day, 18:09:00 STD>

In [12]: pd.DatetimeTZDtype(tz=idx[0].tz).tz
Out[12]: <DstTzInfo 'US/Central' CST-1 day, 18:00:00 STD>
```

would be equal to the normalized version, `Out[11]`.

originally suggested by @jbrockmendel. cc @mroeschke.