Unpin pytest
Follow up from #26619. Since pytest have just done a point release for 4.6.1 we should be good to remove the pin.

https://github.com/pytest-dev/pytest/issues/5358

cc @simonjayhawkins 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26625?src=pr&el=h1) Report
> Merging [#26625](https://codecov.io/gh/pandas-dev/pandas/pull/26625?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/635458029e11ff6d94e8132577075269fb79832c?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/26625/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26625?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26625      +/-   ##
==========================================
- Coverage   91.87%   91.87%   -0.01%     
==========================================
  Files         174      174              
  Lines       50692    50692              
==========================================
- Hits        46575    46571       -4     
- Misses       4117     4121       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.41% <ø> (ø)` | :arrow_up: |
| #single | `41.78% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26625?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26625/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26625/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26625?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/26625?src=pr&el=footer). Last update [6354580...d2beb74](https://codecov.io/gh/pandas-dev/pandas/pull/26625?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/26625?src=pr&el=h1) Report
> Merging [#26625](https://codecov.io/gh/pandas-dev/pandas/pull/26625?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/635458029e11ff6d94e8132577075269fb79832c?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/26625/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26625?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26625      +/-   ##
==========================================
- Coverage   91.88%   91.87%   -0.01%     
==========================================
  Files         174      174              
  Lines       50692    50692              
==========================================
- Hits        46576    46572       -4     
- Misses       4116     4120       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.41% <ø> (ø)` | :arrow_up: |
| #single | `41.8% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26625?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26625/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26625/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26625?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/26625?src=pr&el=footer). Last update [6354580...08c8159](https://codecov.io/gh/pandas-dev/pandas/pull/26625?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Interestingly I think there still might be an issue with the pytest release based on:

```
   def manual_compare_stacked(df, df_stacked, lev0, lev1):
>       assert all(df.loc[row, col] ==
                   df_stacked.loc[(row, col[lev0]), col[lev1]]
                   for row in df.index for col in df.columns)
E       NameError: name 'col' is not defined
```
will take a further look tomorrow bumped up to latest pytest release which should fix this - CI running.. @jreback : How does it work when it comes to developer dependencies?  Should we be bumping version requirements this high to be the latest version? doesn’t make sense to bump this - we try to move very slowly in things like thus Given that pandas tests will fail running with pytest version 4.6.0 and 4.6.1 doesn’t it make sense to enforce this in the dependencies? ( yes we could do this by keeping the current pin in place ).

But keeping it pinned means nothing in CI will draw our attention to any future breaking pytest changes?  Potentially making it harder to update further down the road. What do you think ?  these are 3.5 builds only - pinning is ok

to be honest could pin them to a much earlier version
say 4.02 would be good (or for that would be ok)

all other builds float to the latest