Changes needed in https://github.com/RustPython/RustPython/blob/master/vm/src/vm.rs
Similar to what was done in #197, remaining operations (add, div, pow...) need to call call_or_unsupported.

For example replace

self.call_method(&a, "__truediv__", vec![b])

with

self.call_or_unsupported(a, b, "__truediv__", "__rtruediv__", "/")