TST: flaky test test_agg_consistency on Python 3.5
The following tests fails from time to time on Python 3.5 builds

```
=================================== FAILURES ===================================
_____________________________ test_agg_consistency _____________________________
[gw0] darwin -- Python 3.5.6 /Users/vsts/miniconda3/envs/pandas-dev/bin/python

    def test_agg_consistency():
    
        # make sure that we are consistent across
        # similar aggregations with and w/o selection list
        df = DataFrame(np.random.randn(1000, 3),
                       index=pd.date_range('1/1/2012', freq='S', periods=1000),
                       columns=['A', 'B', 'C'])
    
        r = df.resample('3T')
    
        with tm.assert_produces_warning(FutureWarning,
                                        check_stacklevel=False):
            expected = r[['A', 'B', 'C']].agg({'r1': 'mean', 'r2': 'sum'})
            result = r.agg({'r1': 'mean', 'r2': 'sum'})
>       assert_frame_equal(result, expected)

pandas/tests/resample/test_resample_api.py:250: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pandas/_libs/testing.pyx:65: in pandas._libs.testing.assert_almost_equal
    cpdef assert_almost_equal(a, b,
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   raise_assert_detail(obj, msg, lobj, robj)
E   AssertionError: MultiIndex level [0] are different
E   
E   MultiIndex level [0] values are different (100.0 %)
E   [left]:  Index(['r2', 'r2', 'r2', 'r1', 'r1', 'r1'], dtype='object')
E   [right]: Index(['r1', 'r1', 'r1', 'r2', 'r2', 'r2'], dtype='object')

pandas/_libs/testing.pyx:178: AssertionError
```

See eg https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=13625&view=logs&jobId=521b7dfd-2989-5ff8-bc8c-7481906480fa