CI: Fixed deprecationWarning
Closes #20479

Hopefully. I can't reproduce locally.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20489?src=pr&el=h1) Report
> Merging [#20489](https://codecov.io/gh/pandas-dev/pandas/pull/20489?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6c0c27730fb5b20978824fb380449cd3acef55f7?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20489/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20489?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20489      +/-   ##
==========================================
+ Coverage   91.82%   91.84%   +0.02%     
==========================================
  Files         152      152              
  Lines       49235    49235              
==========================================
+ Hits        45212    45222      +10     
+ Misses       4023     4013      -10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.88% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20489?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20489/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.52% <0%> (-0.21%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20489/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.81% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20489?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/20489?src=pr&el=footer). Last update [6c0c277...6f08858](https://codecov.io/gh/pandas-dev/pandas/pull/20489?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Seems to be fixed in https://travis-ci.org/pandas-dev/pandas/jobs/358376504.