CLN: move PeriodIndex.__getitem__ to DatetimeIndexOpsMixin
- [x] related to #6469
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

merge `PeriodIndex.__getitem__` to `DatetimeIndexOpsMixin`

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13987?src=pr) is 85.27% (diff: 100%)

> Merging [#13987](https://codecov.io/gh/pydata/pandas/pull/13987?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13987   diff @@
==========================================
  Files           139        139          
  Lines         50214      50208     -6   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42822      42816     -6   
  Misses         7392       7392          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [a0d05db...48d187d](https://codecov.io/gh/pydata/pandas/compare/a0d05dbf7477df863486d82a0cdd4e3e93023864...48d187d9d84cfb445f526dc20943aaf1f1ff4609?src=pr)
 thanks!
