TST: Avoid DeprecationWarnings
Avoid a boatload of these:

```
pandas/tests/plotting/test_frame.py::TestDataFramePlots::()::test_plot
  /home/travis/build/pandas-dev/pandas/pandas/core/indexes/multi.py:1556: DeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
    if lab[key] == -1:

  /home/travis/build/pandas-dev/pandas/pandas/io/formats/style.py:1039: DeprecationWarning: In future, it will be an error for 'np.bool_' scalars to be interpreted as an index
    return css_bar(min(x, zero), max(x, zero), colors[x > zero])

pandas/tests/series/test_analytics.py::TestCategoricalSeriesAnalytics::()::test_drop_duplicates_categorical_non_bool[False-datetime64[D]]
  source:2442: DeprecationWarning: invalid escape sequence \(
```

That last one was a nice little adventure tracking down to pd.util.testing.

Does not silence these, as I'm not confident that is The Right Thing To Do:
```
pandas/tests/tslibs/test_parsing.py::TestGuessDatetimeFormat::()::test_guess_datetime_format_nopadding[2011-1-1 0:0:0-%Y-%m-%d %H:%M:%S]
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/dateutil/parser/__init__.py:46: DeprecationWarning: _timelex is a private class and may break without warning, it will be moved and or renamed in future versions.
    warnings.warn(msg, DeprecationWarning)
```

Does not silence these, as I'm not sure how:
```
pandas/tests/io/test_sql.py::TestSQLiteAlchemyConn::()::test_read_table
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/_pytest/fixtures.py:795: RemovedInPytest4Warning: Fixture setup_method called directly. Fixtures are not meant to be called directly, are created automatically when test functions request them as parameters. See https://docs.pytest.org/en/latest/fixture.html for more information.
    res = next(it)
```
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/indexes/base.py`](https://github.com/jbrockmendel/pandas/blob/ac5afd69df6e955950c201ec3c7fd1c90019decb/pandas/core/indexes/base.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/multi.py`](https://github.com/jbrockmendel/pandas/blob/ac5afd69df6e955950c201ec3c7fd1c90019decb/pandas/core/indexes/multi.py) !

 - There are no PEP8 issues in the file [`pandas/io/formats/style.py`](https://github.com/jbrockmendel/pandas/blob/ac5afd69df6e955950c201ec3c7fd1c90019decb/pandas/io/formats/style.py) !

 - There are no PEP8 issues in the file [`pandas/util/testing.py`](https://github.com/jbrockmendel/pandas/blob/ac5afd69df6e955950c201ec3c7fd1c90019decb/pandas/util/testing.py) !

 - Complete extra results for this file :

> file_to_check.py:2442:-258: W605 invalid escape sequence '\('
file_to_check.py:2442:-255: W605 invalid escape sequence '\)'
---

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22646?src=pr&el=h1) Report
> Merging [#22646](https://codecov.io/gh/pandas-dev/pandas/pull/22646?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0976e1261781153603064eb67d66630a099edcd7?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/22646/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22646?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22646      +/-   ##
==========================================
+ Coverage   92.17%   92.17%   +<.01%     
==========================================
  Files         169      169              
  Lines       50708    50715       +7     
==========================================
+ Hits        46740    46747       +7     
  Misses       3968     3968
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.58% <100%> (ø)` | :arrow_up: |
| #single | `42.35% <55.55%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22646?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22646/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.96% <ø> (ø)` | :arrow_up: |
| [pandas/io/formats/style.py](https://codecov.io/gh/pandas-dev/pandas/pull/22646/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvc3R5bGUucHk=) | `96.43% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/22646/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.41% <100%> (ø)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/22646/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `97.39% <100%> (+0.05%)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/22646/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.45% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22646?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/22646?src=pr&el=footer). Last update [0976e12...162962e](https://codecov.io/gh/pandas-dev/pandas/pull/22646?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks! some more if you have a chance :>