[fix] Fix a mypy error

Author: nabenabe0928

autoPyTorch/data/tabular_feature_validator.py

@@ -143,6 +143,7 @@ def _encode_categories(self, X: pd.DataFrame) -> None:
         self.column_transformer.fit(X)
 
         # The column transformer moves categoricals to the left side
+        assert self.feat_type is not None  # mypy definition
         self.feat_type = sorted(self.feat_type, key=functools.cmp_to_key(self._comparator))
 
         encoded_categories = self.column_transformer.\