[python-package] fix mypy error in Booster.get_split_value_histogram() (#5794)

python-package/lightgbm/basic.py

@@ -4415,7 +4415,7 @@ class Booster:
         model = self.dump_model()
         feature_names = model.get('feature_names')
         tree_infos = model['tree_info']
-        values = []
+        values: List[float] = []
         for tree_info in tree_infos:
             add(tree_info['tree_structure'])