BUG: ufunc with PeriodIndex may raise IncompatibleFrequency
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

`PeriodIndex` supportds addition / subtraction with integer.

```
# OK
pi = pd.PeriodIndex(['2011-01'], freq='M')
pi + 1
# PeriodIndex(['2011-02'], dtype='int64', freq='M')
pi - 1
# PeriodIndex(['2010-12'], dtype='int64', freq='M')
```

but raises if op is performed via ufunc.

```
# NG
np.add(pi, 1)
# pandas._period.IncompatibleFrequency: Input has different freq from PeriodIndex(freq=M)

np.subtract(pi, 1)
# pandas._period.IncompatibleFrequency: Input has different freq from PeriodIndex(freq=M)
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13980?src=pr) is 85.26% (diff: 54.54%)

> No coverage report found for **master** at 1919e26.
> 
> Powered by [Codecov](https://codecov.io?src=pr). Last update [1919e26...f94d027](https://codecov.io/gh/pydata/pandas/compare/1919e26ead5d156c2b505a0ad8d233b02eb1b573...f94d0271dea2d4808743493421da989f91231a68?src=pr)
 thanks!

can revisit #13988 with these changes.
