TST: Test cleanup, parametrization for datetime64 arithmetic tests
Avoids overlap with #23642.
Hello @jbrockmendel! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimes.py`](https://github.com/jbrockmendel/pandas/blob/f4775d255c9ca49891fdc7a5c9b3e9233ded8f90/pandas/core/arrays/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_datetime64.py`](https://github.com/jbrockmendel/pandas/blob/f4775d255c9ca49891fdc7a5c9b3e9233ded8f90/pandas/tests/arithmetic/test_datetime64.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/period/test_period.py`](https://github.com/jbrockmendel/pandas/blob/f4775d255c9ca49891fdc7a5c9b3e9233ded8f90/pandas/tests/scalar/period/test_period.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/test_nat.py`](https://github.com/jbrockmendel/pandas/blob/f4775d255c9ca49891fdc7a5c9b3e9233ded8f90/pandas/tests/scalar/test_nat.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timedelta/test_arithmetic.py`](https://github.com/jbrockmendel/pandas/blob/f4775d255c9ca49891fdc7a5c9b3e9233ded8f90/pandas/tests/scalar/timedelta/test_arithmetic.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timestamp/test_comparisons.py`](https://github.com/jbrockmendel/pandas/blob/f4775d255c9ca49891fdc7a5c9b3e9233ded8f90/pandas/tests/scalar/timestamp/test_comparisons.py) !



##### Comment last updated on November 14, 2018 at 01:59 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23681?src=pr&el=h1) Report
> Merging [#23681](https://codecov.io/gh/pandas-dev/pandas/pull/23681?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a23f90142352b6427916292b2b9faa7bb61d07bc?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/23681/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23681?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23681      +/-   ##
==========================================
+ Coverage   92.25%   92.25%   +<.01%     
==========================================
  Files         161      161              
  Lines       51383    51385       +2     
==========================================
+ Hits        47404    47406       +2     
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <100%> (ø)` | :arrow_up: |
| #single | `42.31% <80%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23681?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23681/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `98.48% <100%> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23681?src=pr&el=footer). Last update [a23f901...ddb1662](https://codecov.io/gh/pandas-dev/pandas/pull/23681?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Possible segfault in test_rank? I think this was changed s few days ago gentle ping thanks @jbrockmendel 

seems 2.7 build is failing on last merge @TomAugspurger ? That's the rank segfault being tracked in a separate issue. Will look into
it today.

On Fri, Nov 16, 2018 at 10:24 AM Jeff Reback <notifications@github.com>
wrote:

> thanks @jbrockmendel <https://github.com/jbrockmendel>
>
> seems 2.7 build is failing on last merge @TomAugspurger
> <https://github.com/TomAugspurger> ?
>
> —
> You are receiving this because you were mentioned.
> Reply to this email directly, view it on GitHub
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