CI: Unpin NumPy
Closes https://github.com/pandas-dev/pandas/issues/23172
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/groupby/generic.py`](https://github.com/TomAugspurger/pandas/blob/7ddef8e5c492d37c810b6b0142e969e22e8ee07e/pandas/core/groupby/generic.py) !

 Scipy import error on travis # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23465?src=pr&el=h1) Report
> Merging [#23465](https://codecov.io/gh/pandas-dev/pandas/pull/23465?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0bb24b7ac5c58bf9115485bba8c8b97442ab5059?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23465/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23465?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23465      +/-   ##
==========================================
+ Coverage   92.23%   92.25%   +0.02%     
==========================================
  Files         161      161              
  Lines       51197    51176      -21     
==========================================
- Hits        47220    47214       -6     
+ Misses       3977     3962      -15
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <100%> (+0.02%)` | :arrow_up: |
| #single | `42.28% <33.33%> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23465?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/\_test\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/23465/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3Rlc3RfZGVjb3JhdG9ycy5weQ==) | `93.24% <100%> (ø)` | :arrow_up: |
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/23465/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `87.04% <100%> (ø)` | :arrow_up: |
| [pandas/io/parquet.py](https://codecov.io/gh/pandas-dev/pandas/pull/23465/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnF1ZXQucHk=) | `83.5% <0%> (+9.77%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23465?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/23465?src=pr&el=footer). Last update [0bb24b7...3f84fb0](https://codecov.io/gh/pandas-dev/pandas/pull/23465?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I don't really understand the scipy failure. I can't reproduce it locally right now.

I pushed another commit to see if it fails again. OK, able to reproduce locally (on a Mac) with the latest wheels from https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com:

```
$ python -W error
Python 3.7.0 (default, Sep 18 2018, 18:47:22)
[Clang 9.1.0 (clang-902.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import scipy.signal
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/taugspurger/Envs/pandas-dev/lib/python3.7/site-packages/scipy/signal/__init__.py", line 322, in <module>
    from .filter_design import *
  File "/Users/taugspurger/Envs/pandas-dev/lib/python3.7/site-packages/scipy/signal/filter_design.py", line 18, in <module>
    from scipy import special, optimize, fftpack
  File "/Users/taugspurger/Envs/pandas-dev/lib/python3.7/site-packages/scipy/optimize/__init__.py", line 369, in <module>
    from ._minimize import *
  File "/Users/taugspurger/Envs/pandas-dev/lib/python3.7/site-packages/scipy/optimize/_minimize.py", line 30, in <module>
    from ._trustregion_krylov import _minimize_trust_krylov
  File "/Users/taugspurger/Envs/pandas-dev/lib/python3.7/site-packages/scipy/optimize/_trustregion_krylov.py", line 2, in <module>
    from ._trlib import (get_trlib_quadratic_subproblem)
  File "/Users/taugspurger/Envs/pandas-dev/lib/python3.7/site-packages/scipy/optimize/_trlib/__init__.py", line 1, in <module>
    from ._trlib import TRLIBQuadraticSubproblem
  File "messagestream.pxd", line 5, in init scipy.optimize._trlib._trlib
  File "messagestream.pyx", line 5, in init scipy._lib.messagestream
ImportWarning: can't resolve package from __spec__ or __package__, falling back on __name__ and __path__

>>> import numpy
>>> import scipy
>>> numpy.__version__
'1.16.0.dev0+9cc9f01'
>>> scipy.__version__
'1.2.0.dev0+016a6ef'
```

@rgommers do you know where the configuration for the scipy wheels uploaded to https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com lives?  Or do you know offhand what versionsof NumPy and Cython those are compiled with? > @rgommers do you know where the configuration for the scipy wheels uploaded to https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com lives? Or do you know offhand what versionsof NumPy and Cython those are compiled with?

That should be https://github.com/MacPython/scipy-wheels#how-it-works. Haven't checked, but it should be using the lowest supported NumPy version (same as for official releases, for ABI compatibility). Cython version will all be the same, not sure which version (check CI scripts). Thanks for the pointer.

From https://travis-ci.org/MacPython/scipy-wheels/jobs/432291485, it seems like Cython 0.26.1 and NumPy 1.14.5 are used.

And FWIW, I get the same warning / error with from `python -W error -c "import scipy.signal"` using the currently released versions of NumPy & SciPy as I do with the dev versions, so I'll just assume that's expected for now (though I think Cython 0.29 fixed this). @jreback if you have any thoughts, otherwise I'll merge tomorrow morning so we're testing against numpy master again. lgtm.  thanks!