TST: more 3.6 test fixing
xref #14679 
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14684?src=pr) is 85.28% (diff: 80.00%)

> Merging [#14684](https://codecov.io/gh/pandas-dev/pandas/pull/14684?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14684   diff @@
==========================================
  Files           140        140          
  Lines         50706      50717    +11   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43247      43256     +9   
- Misses         7459       7461     +2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [748000d...66aca46](https://codecov.io/gh/pandas-dev/pandas/compare/748000d220c3c7d4daddf5811e10a5acdfce49ea...66aca467b3aa3eaee8d89fc7f2a4a7838e2ae92c?src=pr)
