Fix TimedeltaIndex +/- offset array
No issue specific to this, but analogus to #18849.  There is a checkbox for this in #18824.

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

After this we'll be able to get rid of _TimeOp entirely # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19095?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@856c92b`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `88.23%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19095/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19095?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19095   +/-   ##
=========================================
  Coverage          ?   91.51%           
=========================================
  Files             ?      148           
  Lines             ?    48778           
  Branches          ?        0           
=========================================
  Hits              ?    44639           
  Misses            ?     4139           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.88% <88.23%> (?)` | |
| #single | `41.61% <14.7%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19095?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/19095/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.08% <100%> (ø)` | |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/19095/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.58% <84.61%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19095?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/19095?src=pr&el=footer). Last update [856c92b...b9e9fff](https://codecov.io/gh/pandas-dev/pandas/pull/19095?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Travis error is a timeout; should I re-push? i restarted Http error on appveyor Ping thanks!