Multi-Metric Evaluation with DistGridSearchCV results in NameError

[davcem]

Describe the bug
Using multi-metric scoring in DistGridSearchCV results in an NameError:
File "/home//.local/lib/python3.6/site-packages/skdist/distribute/search.py", line 315, in fit
not isinstance(self.refit, six.string_types) or
NameError: name 'six' is not defined

To Reproduce
Steps to reproduce the behavior:
Create a DistGridSearchCV:

GS_EVALUATION_METRICS_DICT = {
'accuracy' : 'accuracy',
'roc_auc' : 'roc_auc'
}

model = GaussianNB()
model_param_grid: {'var_smoothing': [1e-08, 0.0001, 0.01]}

grid_search = DistGridSearchCV(estimator=model, 
                           param_grid=model_param_grid,
                           sc=sc, 
                           scoring=GS_EVALUATION_METRICS_DICT, 
                           n_jobs=6, 
                           pre_dispatch=6,
                           cv=3, 
                           refit='roc_auc',
                           verbose=1,
                           error_score=0,
                           return_train_score=True,
                           )

Expected behavior
A clear and concise description of what you expected to happen.
No NameError

Additional context
I think the error is easily fixable --> Add the import of the six library

[denver1117]

Thanks for raising this issue. Would you be open to submitting a PR for this fix and adding test coverage? I'd happily and quickly approve a PR that resolves it. Thanks.