myTree = {bestFeature: {'<' + str(bestSplitValue): {}, '>' + str(bestSplitValue): {}}}
改为
myTree = {bestFeature: {'<' + format(bestSplitValue): {}, '>' + format(bestSplitValue): {}}}

看这里
https://stackoverflow.com/questions/27860725/typeerror-str-object-is-not-callable