TST: skip assert for mpl 3.1+
Closes #26714 
xref https://github.com/pandas-dev/pandas/issues/26716
I'm going offline for at least a few hours, if anyone wants to keep pushing stuff here then feel free.

https://github.com/pandas-dev/pandas/pull/26715/commits/1b011d2f998cd1d8e4658fff8095f78c3357e437 is worth keeping (assuming it works for our oldest MPL)

The other two commits are just debugging. In https://travis-ci.org/pandas-dev/pandas/jobs/542971521#L3018 we see that `purest ---collect-only pandas` is segfaulting. Something is going wrong when we import feather: https://travis-ci.org/pandas-dev/pandas/jobs/542977519#L3052

I'm going to remove it from the env for now, just to get the CI green again. Also, `faulthandler` / `pytest-faulthandler` is very useful. Seems like tests are collecting. Merging on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26715?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@ef30df8`). [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/26715/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26715?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #26715   +/-   ##
=========================================
  Coverage          ?   91.77%           
=========================================
  Files             ?      174           
  Lines             ?    50703           
  Branches          ?        0           
=========================================
  Hits              ?    46534           
  Misses            ?     4169           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.37% <ø> (?)` | |
| #single | `41.81% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26715?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/26715?src=pr&el=footer). Last update [ef30df8...35d058e](https://codecov.io/gh/pandas-dev/pandas/pull/26715?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/26715?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@ef30df8`). [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/26715/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26715?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #26715   +/-   ##
=========================================
  Coverage          ?   91.77%           
=========================================
  Files             ?      174           
  Lines             ?    50703           
  Branches          ?        0           
=========================================
  Hits              ?    46535           
  Misses            ?     4168           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.37% <ø> (?)` | |
| #single | `41.83% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26715?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/26715?src=pr&el=footer). Last update [ef30df8...460b8eb](https://codecov.io/gh/pandas-dev/pandas/pull/26715?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
