added estimator_type property

[perib]

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What does this PR do?

adds a _estimator_type property to both tpotestimators to make it consistent with other sklearn estimators.

@property
def _estimator_type(self):
    return self.fitted_pipeline_._estimator_type

Any background context you want to provide?

After the new autosklearn update, this property used by some scorers to determine whether the pipeline was a classifier or regressor. These scorers would throw an error when used with either tpotestimators.