Fix+test DTI/TDI/PI add/sub with ndarray[datetime64/timedelta64]
Two notes for upcoming passes:

- the tests are pretty redundant, can be parametrized more tightly once they are all OK.
- catching ndarray[datetime64] separately from DatetimeIndex is kind of weird, but for the moment necesarry.  Once #19835 goes through and re-orders the catching, we'll be able to fix this.
@jbrockmendel : Good direction so far!  Looks you can parametrize now since all the CI is green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19847?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@e97be6f`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19847/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19847?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19847   +/-   ##
=========================================
  Coverage          ?   91.69%           
=========================================
  Files             ?      150           
  Lines             ?    48920           
  Branches          ?        0           
=========================================
  Hits              ?    44857           
  Misses            ?     4063           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.07% <100%> (?)` | |
| #single | `41.82% <8.33%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19847?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/19847/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.99% <100%> (ø)` | |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/19847/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.38% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19847?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/19847?src=pr&el=footer). Last update [e97be6f...dc67762](https://codecov.io/gh/pandas-dev/pandas/pull/19847?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 needs a rebase. does this fix any open issue? > does this fix any open issue?

Not that I'm aware of. Here too Travis error is pyarrow import thanks