TST/PKG: Remove definition of pandas.util.testing.slow
And just use @pytest.mark.slow instead

closes #16850
We may want to start using the `--strict` flag as well. It will catch typos like `@pytest.mark.slwo` # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=h1) Report
> Merging [#16852](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/500cd0f1935d89c64d93f42cbf373a1209870600?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16852/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16852      +/-   ##
==========================================
- Coverage   90.99%   90.97%   -0.02%     
==========================================
  Files         161      161              
  Lines       49291    49291              
==========================================
- Hits        44850    44842       -8     
- Misses       4441     4449       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.74% <ø> (ø)` | :arrow_up: |
| #single | `40.16% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `100% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.33% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16852?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/16852?src=pr&el=footer). Last update [500cd0f...8f60442](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=h1) Report
> Merging [#16852](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/500cd0f1935d89c64d93f42cbf373a1209870600?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16852/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16852      +/-   ##
==========================================
- Coverage   90.99%   90.97%   -0.02%     
==========================================
  Files         161      161              
  Lines       49291    49291              
==========================================
- Hits        44850    44842       -8     
- Misses       4441     4449       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.74% <ø> (ø)` | :arrow_up: |
| #single | `40.16% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `100% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.33% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16852?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/16852?src=pr&el=footer). Last update [500cd0f...8f60442](https://codecov.io/gh/pandas-dev/pandas/pull/16852?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. thanks @TomAugspurger  (if you want to add --strict in another PR go for it) I just ran the test suite of the downstream package that uncovered this problem in the first place and I can confirm that it passes all test now. Thanks for fixing this! Thank you for checking on it!

On Mon, Jul 17, 2017 at 8:49 AM, Maarten Derickx <notifications@github.com>
wrote:

> I just tested the test suite of the downstream package that uncovered this
> problem in the first place and I can confirm that it passes all test now.
> Thanks for fixing this!
>
> —
> You are receiving this because you were mentioned.
> Reply to this email directly, view it on GitHub
> <https://github.com/pandas-dev/pandas/pull/16852#issuecomment-315760886>,
> or mute the thread
> <https://github.com/notifications/unsubscribe-auth/ABQHIoL5EaQWnLRnW2GKv60ukqH_afAmks5sO2ZNgaJpZM4OQ238>
> .
>
