Fix Series.__sub__ non-nano datetime64
The original bug report was for `Series`.  This fixes that bug and includes a test that checks for `DatetimeIndex` while we're at it.  I checked and this does _not_ fix the analogous problem in `DataFrame.  I'm hoping someone else will pick up the torch on that b/c the broadcast/dispatch is still something of a mystery to me.

- [x] closes #7996
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18783?src=pr&el=h1) Report
> Merging [#18783](https://codecov.io/gh/pandas-dev/pandas/pull/18783?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cdebcf374598e9141df7dc23bc75c9528e92c004?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18783/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18783?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18783      +/-   ##
==========================================
+ Coverage   91.59%   91.59%   +<.01%     
==========================================
  Files         150      150              
  Lines       48959    48961       +2     
==========================================
+ Hits        44843    44847       +4     
+ Misses       4116     4114       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.96% <100%> (ø)` | :arrow_up: |
| #single | `41.13% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18783?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/18783/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `90.2% <100%> (+0.03%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18783/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.9% <0%> (+0.21%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18783?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18783?src=pr&el=footer). Last update [cdebcf3...0dbe62c](https://codecov.io/gh/pandas-dev/pandas/pull/18783?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Timeout on circleCI in collection, test_geopandas in travis.  Will wait to re-push until given the go-ahead. yeah don't re-push anything. Ping This somehow doesn't need a whatsnew merge thanks!