@serhiy.storchaka, while a properly working function that uses getrandbits isn't so complex, its not trivial either.

It needs to create smaller chunks and join them (also check zero size case which raises an error if passed).

eg:

```
    def urandom_from_random(rng, length):
        if length == 0:
            return b''
    
        import sys
        chunk_size = 65535
        chunks = []
        while length >= chunk_size:
            chunks.append(rng.getrandbits(chunk_size * 8).to_bytes(chunk_size, sys.byteorder))
            length -= chunk_size
        if length:
            chunks.append(rng.getrandbits(length * 8).to_bytes(length, sys.byteorder))
        result = b''.join(chunks)
        return result
```