BUG: fixed index power operation
* The power operation on a range of indexes was fixed. See issue #14973

 - [ ] closes #14973
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14996?src=pr) is 84.78% (diff: 100%)
> Merging [#14996](https://codecov.io/gh/pandas-dev/pandas/pull/14996?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14996   diff @@
==========================================
  Files           145        145          
  Lines         51090      51091     +1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43315      43317     +2   
+ Misses         7775       7774     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [7f0eefc...66c2338](https://codecov.io/gh/pandas-dev/pandas/compare/7f0eefc2f81d6b34f252a5da38d31e515c295106...66c23384c657047308b9c4eac4f593309216d84e?src=pr) @jreback  anything more I can do to have it accepted? thanks!