TST: Test timedelta arithmetic (#9396)
 - [x] closes #9396
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

Original issue was with using `div` with `datetime.timedelta`.  Doesn't appear this was fixed recently.  
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14906?src=pr) is 84.56% (diff: 100%)
> Merging [#14906](https://codecov.io/gh/pandas-dev/pandas/pull/14906?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #14906   diff @@
==========================================
  Files           144        144          
  Lines         51058      51058          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43177      43176     -1   
- Misses         7881       7882     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [e503d40...56c5ba3](https://codecov.io/gh/pandas-dev/pandas/compare/e503d40ace473556990e5453ed5b4c9aa96e24ff...56c5ba37c270c5e2f02932f7c21a1edfe1f4d461?src=pr) thanks!