[Python] Accept bytes object in pyarrow.substrait.run_query

pyarrow.substrait.run_query() only accepts a PyArrow buffer, and will segfault if something else is passed. People might try to pass a Python bytes object, which isn't unreasonable. For example, they might use the value returned by protobufs SerializeToString() function, which is Python bytes. At the very least, we should not segfault.