Add kernelcreation.make_python_function (generic version of...

Add kernelcreation.make_python_function (generic version of {cuda,cpu,llvm}jit.make_python_function)

This is roughly equivalent to kernel_function_node.compile(backend=...)
but it is easier to find and inspect.
Since all backend have `make_python_function` it makes sense to add a
generic one.