BUG: DatetimeTz shift raises AmbiguousTimeError near DST

[sinhrks]

• tests added / passed
• passes git diff upstream/master | flake8 --diff
• 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

[codecov-io]

Current coverage is 85.30% (diff: 100%)

Merging #13926 into master will increase coverage by <.01%

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[jreback]

ty sir!