BUG: Addition raises TypeError if Period is on rhs
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

`Period` addition raises `TypeError` if `Period` is on right hand side.

```
1 + pd.Period('2011-01', freq='M')
# TypeError: unsupported operand type(s) for +: 'int' and 'pandas._period.Period'
```
#### Expected

```
1 + pd.Period('2011-01', freq='M')
# Period('2011-02', 'M')
```

didn't you make this an issue somewhere? (or maybe just a comment)
 No existing issue. Updated the top comment.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13069?src=pr) is **84.14%**

> Merging [#13069](https://codecov.io/gh/pydata/pandas/pull/13069?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **+<.01%**

``` diff
@@             master     #13069   diff @@
==========================================
  Files           137        137          
  Lines         50261      50231    -30   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42288      42265    -23   
+ Misses         7973       7966     -7   
  Partials          0          0          
```
1. 2 files (not in diff) in `pandas/tseries` were modified. [more](https://codecov.io/gh/pydata/pandas/commit/cdcd3b35836955b1205443f7d0c6edd3d93f7c3e/changes?src=pr#70616E6461732F74736572696573) 
   - Misses `-4` 
   - Hits `-22`
2. 2 files (not in diff) in `pandas` were modified. [more](https://codecov.io/gh/pydata/pandas/commit/cdcd3b35836955b1205443f7d0c6edd3d93f7c3e/changes?src=pr#70616E646173) 
   - Misses `-3` 
   - Hits `-5`

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [1296ab3...cdcd3b3](https://codecov.io/gh/pydata/pandas/compare/1296ab39d32acaf2c77ed0c185fafe4ebcfedcb3...cdcd3b35836955b1205443f7d0c6edd3d93f7c3e)
 thanks @sinhrks 
