BUG: AttributeError for Interaction Values with RandomForestClassifier

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Issue Description

When calculating the interaction values for the RandomForestClassifier one runs into an AttributeError.

Minimal Reproducible Example

import shap
from sklearn.ensemble import RandomForestClassifier

from shap.explainers import TreeExplainer

X, y = shap.datasets.adult(n_points=50)
rfc = RandomForestClassifier(max_depth=1).fit(X, y)
ex_rfc = TreeExplainer(rfc)
e_rfc = ex_rfc(X, interactions=True)

Traceback

Traceback (most recent call last):
  File "/home/tobias/programming/github/shap/bugs/bug_report.py", line 16, in <module>
    e_rfc = ex_rfc(X, interactions=True)
  File "/home/tobias/programming/github/shap/shap/explainers/_tree.py", line 244, in __call__
    ev_tiled = np.tile(self.expected_value, (v.shape[0], 1))
AttributeError: 'list' object has no attribute 'shape'

Expected Behavior

Output should be consistent with output of XGBoost, CatBoost, etc. See this issue for further discussion on consistency.

Bug report checklist

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest release of shap.
• I have confirmed this bug exists on the master branch of shap.
• I'd be interested in making a PR to fix this bug

Installed Versions

shap-0.43.0