COMPAT: remove NaT comparison warnings with numpy >= 1.11
cc @sinhrks 

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/12969?src=pr) is **83.82%**

> Merging [#12969](https://codecov.io/gh/pydata/pandas/pull/12969?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **-0.00%**

``` diff
@@           master   #12969   diff @@
======================================
  Files         136      136          
  Lines       49642    49663    +21   
  Methods         0        0          
  Branches        0        0          
======================================
+ Hits        41617    41632    +15   
- Misses       8025     8031     +6   
  Partials        0        0          
```

[![Sunburst](https://codecov.io/gh/pydata/pandas/pull/12969/graphs/sunburst.svg?src=pr&size=660)](https://codecov.io/gh/pydata/pandas/pull/12969?src=pr)

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by 5b766d2
