COMPAT: don't rely on dtype being False after numpy >= 1.13
closes #15199


## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15200?src=pr) is 85.97% (diff: 100%)
> Merging [#15200](https://codecov.io/gh/pandas-dev/pandas/pull/15200?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **0.01%**

```diff
@@             master     #15200   diff @@
==========================================
  Files           140        140          
  Lines         51273      51263    -10   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          44091      44075    -16   
- Misses         7182       7188     +6   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [dc40058...69f3fa4](https://codecov.io/gh/pandas-dev/pandas/compare/dc400587a3e792d90f5db0be6caff7a4e12574c0...69f3fa44c37f1edec3b2ac2f7ef00e52fa1eda2c?src=pr)