COMPAT: 3.6.1 compat for change in PySlice_GetIndices_Ex
This doesn't actually matter to any tests except for some internal consistency ones.
Bonus is that it eliminates a warning :<

note that we aren't actually testing this (yet) on Travis as our 3.6 build uses conda-forge and 3.6.1 is not there as of yet. Its in defaults though (and shows up on appveyor build).
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15790?src=pr&el=h1) Report
> Merging [#15790](https://codecov.io/gh/pandas-dev/pandas/pull/15790?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7fa77527c747f2d91b6c16fe512cd05a7a072ec9?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15790      +/-   ##
==========================================
- Coverage   91.01%      91%   -0.02%     
==========================================
  Files         143      143              
  Lines       49400    49400              
==========================================
- Hits        44961    44954       -7     
- Misses       4439     4446       +7
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15790?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/7fa77527c747f2d91b6c16fe512cd05a7a072ec9...42ddddc9dd9f9f402070b5b4fa0ee5dc9668dae5?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/7fa77527c747f2d91b6c16fe512cd05a7a072ec9...42ddddc9dd9f9f402070b5b4fa0ee5dc9668dae5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.86% <0%> (-0.1%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/compare/7fa77527c747f2d91b6c16fe512cd05a7a072ec9...42ddddc9dd9f9f402070b5b4fa0ee5dc9668dae5?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.85% <0%> (+0.18%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15790?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/15790?src=pr&el=footer). Last update [7fa7752...42ddddc](https://codecov.io/gh/pandas-dev/pandas/compare/7fa77527c747f2d91b6c16fe512cd05a7a072ec9...42ddddc9dd9f9f402070b5b4fa0ee5dc9668dae5?src=pr&el=footer&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).