MAINT: Remove unused mock import
We import it, set it as an attribute, and then don't use it.

xref <a href="https://github.com/pandas-dev/pandas/issues/16716#issuecomment-315127204">#16716 (comment)</a>

cc @TomAugspurger 
@jreback : Some unrelated `test_clipboard` failure occurred on Travis.  Could you restart the failing build <a href="https://travis-ci.org/pandas-dev/pandas/jobs/253307819">here</a> just so that everything is green? @gfyoung hah, you *should* be able to restart them now! (you may need to have travis sync things to get the permissions) Well, would you look at that?  I can!  Thanks :smile: # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16908?src=pr&el=h1) Report
> Merging [#16908](https://codecov.io/gh/pandas-dev/pandas/pull/16908?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9d13227345882daaa90f03078c09a9b44a18ce72?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/16908/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16908?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16908      +/-   ##
==========================================
- Coverage   90.99%   90.97%   -0.02%     
==========================================
  Files         161      161              
  Lines       49301    49301              
==========================================
- Hits        44861    44852       -9     
- Misses       4440     4449       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.74% <ø> (ø)` | :arrow_up: |
| #single | `40.19% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16908?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16908?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16908?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16908?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/16908?src=pr&el=footer). Last update [9d13227...b963bfe](https://codecov.io/gh/pandas-dev/pandas/pull/16908?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @gfyoung FYI make sure to tag PR's (with labels), and then when merging they are on the correct milestone. @jreback : Ah, right.  I need to get into that habit now. :smile: 