CLN/CI: Catch that stderr-warning!
- [x] closes one of the three problems in #23680
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

There's an uncaught warning to stderr that bubbles up occasionally without failing the build (as far as I can tell), but it was one of the headfakes on the hunt for the ResourceWarnings in #23680.

There's a long list of failing travis builds in that PR (plus in #22225, #23192, #23582), which *also* suffer from the warning that this PR fixes, which bubbles up at unpredictable times and looks something like this in the output:
```
..............................................................x...........................................................s....
........................................Skipping line 3: Expected 3 fields in line 3, saw 4
.......................................s.......................................................................................
```

Hello @h-vetinari! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/io/parser/python_parser_only.py`](https://github.com/h-vetinari/pandas/blob/f5a4bf77dff9e5437fcc0d7bafe5a1cd1bb730c1/pandas/tests/io/parser/python_parser_only.py) !

 lgtm. ping on green. thanks for tracking down! # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23706?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@e413c49`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23706/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23706?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #23706   +/-   ##
=========================================
  Coverage          ?   92.24%           
=========================================
  Files             ?      161           
  Lines             ?    51339           
  Branches          ?        0           
=========================================
  Hits              ?    47360           
  Misses            ?     3979           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <ø> (?)` | |
| #single | `42.34% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23706?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/23706?src=pr&el=footer). Last update [e413c49...b097f83](https://codecov.io/gh/pandas-dev/pandas/pull/23706?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/23706?src=pr&el=h1) Report
> Merging [#23706](https://codecov.io/gh/pandas-dev/pandas/pull/23706?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e413c491e090274aad78489cc17a2e29cbd8e269?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23706/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23706?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23706   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         161      161           
  Lines       51339    51339           
=======================================
  Hits        47360    47360           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <ø> (ø)` | :arrow_up: |
| #single | `42.34% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23706?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/23706/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.68% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23706?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/23706?src=pr&el=footer). Last update [e413c49...ac5288c](https://codecov.io/gh/pandas-dev/pandas/pull/23706?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I'll take your word for it that this catches that warning; big props for getting rid of that annoying thing. @gfyoung 
> @h-vetinari : FYI, instead of pushing commits to trigger entirely new builds, just ping one of us. We can restart individual builds for you.

Could you retrigger that timed-out travis job please? ;-) thanks