It seems like an esoteric parameter that doesn’t have any value outside of one specific use: number stringification. I think it would confuse readers of str’s type signature (shhh, yes I know about the decoding parameters).
I agree with @EpicWink about this being the wrong place (wrong API). In builtins is where I would expect it, alongside hex(), or in int or the math library.
It seems a curious omission that there is no built-in conversion to arbitrary base that I can find. PyNumber_ToBase (_PyLong_Format really) goes to great lengths to optimise the few bases it supports but there’s not even a slow path if you want 6*9 base 13.