fix MacPython / pandas-wheels ci failures
https://github.com/pandas-dev/pandas/pull/25483#issuecomment-469366672

@jreback : the regex is getting a bit unreadable.. this may suggest that a more (user friendly|consistent) error message should be raised by `safe_sort`?
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25537?src=pr&el=h1) Report
> Merging [#25537](https://codecov.io/gh/pandas-dev/pandas/pull/25537?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3bbcacf95b142776150a235dabb234806d87dff9?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25537/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25537?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25537      +/-   ##
==========================================
- Coverage   91.75%   91.75%   -0.01%     
==========================================
  Files         173      173              
  Lines       52960    52960              
==========================================
- Hits        48596    48595       -1     
- Misses       4364     4365       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.33% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25537?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25537/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.57% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25537?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/25537?src=pr&el=footer). Last update [3bbcacf...72ac8cb](https://codecov.io/gh/pandas-dev/pandas/pull/25537?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/25537?src=pr&el=h1) Report
> Merging [#25537](https://codecov.io/gh/pandas-dev/pandas/pull/25537?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3bbcacf95b142776150a235dabb234806d87dff9?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25537/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25537?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25537      +/-   ##
==========================================
- Coverage   91.75%   91.75%   -0.01%     
==========================================
  Files         173      173              
  Lines       52960    52960              
==========================================
- Hits        48596    48595       -1     
- Misses       4364     4365       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.33% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25537?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25537/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.57% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25537?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/25537?src=pr&el=footer). Last update [3bbcacf...72ac8cb](https://codecov.io/gh/pandas-dev/pandas/pull/25537?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback DO NOT MERGE. looks like i made a mistake when trying to fit on one line! > @jreback DO NOT MERGE. looks like i made a mistake when trying to fit on one line!

false alarm! maybe the line breaks could have been chosen better. > the regex is getting a bit unreadable.. this may suggest that a more (user friendly|consistent) error message should be raised by safe_sort?

yep might be better

ok to merge? > ok to merge?

yes.