try pinning numpy dev
xref #22960 
FYI, planning to merge once https://travis-ci.org/pandas-dev/pandas/jobs/442114100 passes, since this only affects that environment.

edit: and there is goes. Merging. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23178?src=pr&el=h1) Report
> Merging [#23178](https://codecov.io/gh/pandas-dev/pandas/pull/23178?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/aaa69d1bb6ed2539f2dbdba842d83a2f90860aa0?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/23178/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23178?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23178   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         170      170           
  Lines       50963    50963           
=======================================
  Hits        46984    46984           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.29% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23178?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/23178?src=pr&el=footer). Last update [aaa69d1...70c9163](https://codecov.io/gh/pandas-dev/pandas/pull/23178?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Something with `pyarrow` also started failing overnight (the azure Win36 job is failing for all recent builds: https://dev.azure.com/pandas-dev/pandas/_build - also happened for this PR, but for some reason the azure link is not visible in the results anymore; here it is): https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=1251&view=logs