BUG: DatetimeTz shift raises AmbiguousTimeError near DST
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

As the same as #13650, caused by `.tz_localize(None)` which can be removed.

```
import pandas as pd
s = pd.Series(pd.date_range('2016-11-06', freq='H', periods=10, tz='US/Eastern'))
s.shift(1)
# pytz.exceptions.AmbiguousTimeError: Cannot infer dst time from Timestamp('2016-11-06 01:00:00'), try using the 'ambiguous' argument
```
