Duplicate logging in some special cases about backoff

[smagafurov]

Solution (move _config_handlers(...) and _prepare_logger(...) from decorate scope to on_exception scope):

def on_exception(
    ...
    ) -> Callable[[_CallableT], _CallableT]:
    ...
    logger = _prepare_logger(logger)
    on_success = _config_handlers(on_success)
    on_backoff = _config_handlers(
        on_backoff,
        default_handler=_log_backoff,
        logger=logger,
        log_level=backoff_log_level,
    )
    on_giveup = _config_handlers(
        on_giveup,
        default_handler=_log_giveup,
        logger=logger,
        log_level=giveup_log_level,
    )

    def decorate(target):
        if asyncio.iscoroutinefunction(target):
            retry = _async.retry_exception
        else:
            retry = _sync.retry_exception
        ...