bd = {b'password': [b'passwd'],
      b'username': [b'testusername', {b'test': b'testval'}]}

def stringify(obj):
    if hasattr(obj, 'items'):
        # dict
        ret = {}
        for k, v in obj.items():
            ret[stringify(k)] = stringify(v)
        return ret

    elif isinstance(obj, bytes):
        # bytes
        return obj.decode()

    elif isinstance(obj, list):
        # list
        return [stringify(x) for x in obj]
    else:
        # anything else
        return obj

d = stringify(bd)