REF/TST: Fix remaining DatetimeArray with DateOffset arithmetic ops
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/66c866bd8e00d5ed1e6f29a9fe6b1dc5eb26ae3e/pandas/core/arrays/timedeltas.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/66c866bd8e00d5ed1e6f29a9fe6b1dc5eb26ae3e/pandas/core/indexes/timedeltas.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_datetime64.py`](https://github.com/jbrockmendel/pandas/blob/66c866bd8e00d5ed1e6f29a9fe6b1dc5eb26ae3e/pandas/tests/arithmetic/test_datetime64.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_timedelta64.py`](https://github.com/jbrockmendel/pandas/blob/66c866bd8e00d5ed1e6f29a9fe6b1dc5eb26ae3e/pandas/tests/arithmetic/test_timedelta64.py) !

 - There are no PEP8 issues in the file [`pandas/tseries/offsets.py`](https://github.com/jbrockmendel/pandas/blob/66c866bd8e00d5ed1e6f29a9fe6b1dc5eb26ae3e/pandas/tseries/offsets.py) !

 Travis fail is the test_rank segfault # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23789?src=pr&el=h1) Report
> Merging [#23789](https://codecov.io/gh/pandas-dev/pandas/pull/23789?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ee283faee006abf5b879d37fe17c2db4e0fa51fd?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `94%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23789/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23789?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23789      +/-   ##
==========================================
- Coverage   92.31%   92.31%   -0.01%     
==========================================
  Files         161      161              
  Lines       51471    51501      +30     
==========================================
+ Hits        47515    47542      +27     
- Misses       3956     3959       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <94%> (-0.01%)` | :arrow_down: |
| #single | `42.43% <36%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23789?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/23789/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `97.08% <100%> (ø)` | :arrow_up: |
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23789/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `96.25% <100%> (-0.19%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23789/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.48% <100%> (+0.01%)` | :arrow_up: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23789/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `89.28% <100%> (+0.09%)` | :arrow_up: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/23789/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.84% <86.36%> (-0.14%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23789?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/23789?src=pr&el=footer). Last update [ee283fa...c3d775e](https://codecov.io/gh/pandas-dev/pandas/pull/23789?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 gentle ping @jorisvandenbossche I think your main concern has been addressed.  LMK if any others are deal-breakers. Should be all squared away now. gentle ping.  This and #23829 are the main things standing in the way of declaring DTA/TDA "finished" (internally) couple of notes for followup, but lgtm.

@jorisvandenbossche  Thanks @jbrockmendel ! Booyah.  Thanks @jorisvandenbossche, this one is a big win.