CI: Test against Python 3.7
- [x] closes #20552 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I will aim to keep as much there as possible. can u update install.rst and setup.py where needed # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21604?src=pr&el=h1) Report
> Merging [#21604](https://codecov.io/gh/pandas-dev/pandas/pull/21604?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7bee353cd69ab846317301fdf614a94fbae50117?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21604/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21604?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21604   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         153      153           
  Lines       49548    49548           
=======================================
  Hits        45538    45538           
  Misses       4010     4010
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <100%> (ø)` | :arrow_up: |
| #single | `41.78% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21604?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/21604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `58.6% <100%> (+0.17%)` | :arrow_up: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/21604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.63% <0%> (-0.02%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21604?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/21604?src=pr&el=footer). Last update [7bee353...de3e8df](https://codecov.io/gh/pandas-dev/pandas/pull/21604?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Merging this, to ensure things don't slip in before the release. Hey, @TomAugspurger 
I see two separate PR failing CI testing running Python3.7 due to the same error:
https://travis-ci.org/pandas-dev/pandas/jobs/396604150
https://travis-ci.org/pandas-dev/pandas/jobs/396594344

Is this expected? 

Thanks,

Alistair