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

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13995?src=pr) is 85.28% (diff: 100%)

> Merging [#13995](https://codecov.io/gh/pydata/pandas/pull/13995?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13995   diff @@
==========================================
  Files           139        139          
  Lines         50206      50210     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42817      42821     +4   
  Misses         7389       7389          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [6f88416...c627549](https://codecov.io/gh/pydata/pandas/compare/6f8841648d068cc04c7bfe0625f131b7c7d6469e...c6275491cbe6fc972e16cb922a3cb7e199f43cec?src=pr)
 thxs!
