`logging.shutdown()` should clear `logging._handlerList` if that's what it's called with

Bug report

Bug description:

In the wild, I've encountered situations where logging.shutdown gets called more than once in a process.

The current implementation is called with logging._handlerList by default, meaning that handlers can have their flush() and close() methods called more than once, which can cause bad things to happen if those handlers are not amenable to being closed more than once.

If logging.shutdown() is processing logging._handlerList, it should clear that list once it finished to prevent handlers having their flush() and close() methods called more than once.

CPython versions tested on:

3.12, CPython main branch

Operating systems tested on:

Linux, macOS

Linked PRs

• gh-129681

从 logging.shutdown() 入口点开始，检查它如何处理 logging._handlerList，包括报告中描述的对 flush() 和 close() 的重复调用。确认 shutdown 在处理默认处理器列表后会清空该列表，并验证重复调用 shutdown 不会再次处理相同的处理器。