BUG : assignment of non-ns timedelta64 value (GH14155)
- [x] closes #14155 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

tests might be in tseries/test_timedelta.py but more likely in index/test_indexing.py
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14160?src=pr) is 85.24% (diff: 100%)

> Merging [#14160](https://codecov.io/gh/pydata/pandas/pull/14160?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14160   diff @@
==========================================
  Files           140        140          
  Lines         50549      50549          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43091      43091          
  Misses         7458       7458          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [1ace12b...dce11f0](https://codecov.io/gh/pydata/pandas/compare/1ace12b7473e6444f7bbd56f90343ca7638570c3...dce11f0c60cdab426532565a45437599e550eb47?src=pr)
 Added the call to other, and added a test in `tests/series/test_indexing.py`.  Please let me know if there is any other comments or changes needed.
 I have edited the `whatsnew` to be a bit clearer, changed `ix` to `loc`, and fixed the problems with PEP8.
 minor comments. lgtm. ping on green.
 @jreback , looks like all checks have passed.  Let me know if there is anything else I can do.
 @rsmith54 Maybe something went wrong with committing the changes, but it seems the `Timedelta(np.timedelta64(other))` has not changed
 lgtm.
 thanks!
