TST: install cython from pip for 3.6_NUMPY_DEV build
@TomAugspurger seems cython released 0.27.0, its working for me locally but seems to break: https://travis-ci.org/MacPython/pandas-wheels/jobs/279039062

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=h1) Report
> Merging [#17657](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/965c1c89b6df471d88dc0e1188fb8cbc0d89f867?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17657/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17657      +/-   ##
==========================================
- Coverage   91.26%   91.22%   -0.05%     
==========================================
  Files         163      163              
  Lines       49806    49806              
==========================================
- Hits        45455    45434      -21     
- Misses       4351     4372      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.3% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17657?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/17657?src=pr&el=footer). Last update [965c1c8...f125c17](https://codecov.io/gh/pandas-dev/pandas/pull/17657?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/17657?src=pr&el=h1) Report
> Merging [#17657](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/965c1c89b6df471d88dc0e1188fb8cbc0d89f867?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17657/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17657      +/-   ##
==========================================
- Coverage   91.26%   91.22%   -0.05%     
==========================================
  Files         163      163              
  Lines       49806    49806              
==========================================
- Hits        45455    45434      -21     
- Misses       4351     4372      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.3% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17657?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/17657?src=pr&el=footer). Last update [965c1c8...f125c17](https://codecov.io/gh/pandas-dev/pandas/pull/17657?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
