backends do not load in proper order

[This line](https://github.com/arrayfire/arrayfire-python/blob/28a0f610c5329ebc718ce4d924f870c3651bfb3e/arrayfire/library.py#L646) has a comment `# Iterate in reverse order of preference`. However, it loads them with some random order, because the object to iterate is a `set`, which doesn't keep the order. In my Linux computer, when I run `import arrayfire as af`, the function will be called and it loads `""` first, and due to `err!=0`, it directly returns and not loads anything.

By the way, I simply change `{"cpu", "opencl", "cuda", ""}` to `["cuda"]` and I can import arrayfire with no error.