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
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13926?src=pr) is 85.30% (diff: 100%)

> Merging [#13926](https://codecov.io/gh/pydata/pandas/pull/13926?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13926   diff @@
==========================================
  Files           139        139          
  Lines         50138      50141     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42769      42773     +4   
+ Misses         7369       7368     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [3186fef...c079ee3](https://codecov.io/gh/pydata/pandas/compare/3186fef545b55ca7b4a3c79c800f32b1d586545e...c079ee3a5cdf6d77e53d647c4122132faf942ee7?src=pr)
 ty sir!
