Bump pytest
- [X] closes #22319
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@TomAugspurger 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22320?src=pr&el=h1) Report
> Merging [#22320](https://codecov.io/gh/pandas-dev/pandas/pull/22320?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/70e6f7c3ce7aca9a0ee08bacb2fe0ad85db02d88?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/22320/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/22320?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22320   +/-   ##
=======================================
  Coverage   92.05%   92.05%           
=======================================
  Files         169      169           
  Lines       50709    50709           
=======================================
  Hits        46679    46679           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.46% <ø> (ø)` | :arrow_up: |
| #single | `42.25% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22320?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/22320?src=pr&el=footer). Last update [70e6f7c...b26dacc](https://codecov.io/gh/pandas-dev/pandas/pull/22320?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 There’s a way to do that directly with pytest. pytest_min_version of something like that.

________________________________
From: codecov[bot] <notifications@github.com>
Sent: Monday, August 13, 2018 7:39:46 PM
To: pandas-dev/pandas
Cc: Tom Augspurger; Mention
Subject: Re: [pandas-dev/pandas] Bump pytest (#22320)

Codecov<https://codecov.io/gh/pandas-dev/pandas/pull/22320?src=pr&el=h1> Report

Merging #22320<https://codecov.io/gh/pandas-dev/pandas/pull/22320?src=pr&el=desc> into master<https://codecov.io/gh/pandas-dev/pandas/commit/601d71fcfa9081fa88e5c20148972cd766396cd7?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/22320?src=pr&el=tree>

@@           Coverage Diff           @@
##           master   #22320   +/-   ##
=======================================
  Coverage   92.08%   92.08%
=======================================
  Files         169      169
  Lines       50706    50706
=======================================
  Hits        46691    46691
  Misses       4015     4015

Flag    Coverage Δ
#multiple       90.49% <ø> (ø)  ⬆️
#single 42.33% <ø> (ø)  ⬆️
________________________________

Continue to review full report at Codecov<https://codecov.io/gh/pandas-dev/pandas/pull/22320?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/22320?src=pr&el=footer>. Last update 601d71f...68941b3<https://codecov.io/gh/pandas-dev/pandas/pull/22320?src=pr&el=lastupdated>. Read the comment docs<https://docs.codecov.io/docs/pull-request-comments>.

—
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 Went with @TomAugspurger suggestion and updated the `setup.cfg` to require this, which gives a nice error message when trying to run below the required version:

```sh
ERROR: /Users/williamayd/clones/pandas/setup.cfg:34: requires pytest-3.6, actual pytest-3.4.1'
```

Not sure if there's a reasonable way to create a test for this so haven't done that, but @jreback if that's a deal breaker lmk and can explore options can you add a whatsnew note (in Development, pls create if we don't have it). We put significant changes in the development process there. *may* need to edit slightly in contributing.rst as well.

This is actually user visible change as well. As users do ``pandas.test()``, so *may* want to actually list in the dependencies changed (even though its not a strict dep).

and maybe add to install.rst as well (I don't think its listed), but for running tests it is required. thanks!