Replace `select`/`poll` with `DefaultSelector`

[MetRonnie]

Follow-up to #7088

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] No dependency changes
• [x] Tests, changelog, docs not needed
• [x] Not a confirmed bug, so raising on master

[MetRonnie]

Got a couple of failures to look into

    Traceback (most recent call last):
      File "/home/runner/work/cylc-flow/cylc-flow/cylc/flow/scheduler_cli.py", line 741, in cylc_play
        asyncio.get_running_loop()
    RuntimeError: no running event loop
    During handling of the above exception, another exception occurred:
    Traceback (most recent call last):
      File "/home/runner/work/cylc-flow/cylc-flow/cylc/flow/scheduler.py", line 711, in run_scheduler
        await self._main_loop()
      File "/home/runner/work/cylc-flow/cylc-flow/cylc/flow/scheduler.py", line 1739, in _main_loop
        await self.workflow_shutdown()
      File "/home/runner/work/cylc-flow/cylc-flow/cylc/flow/scheduler.py", line 1493, in workflow_shutdown
        self.proc_pool.process()
      File "/home/runner/work/cylc-flow/cylc-flow/cylc/flow/subprocpool.py", line 320, in process
        self._poll_proc_pipes(proc, ctx)
      File "/home/runner/work/cylc-flow/cylc-flow/cylc/flow/subprocpool.py", line 443, in _poll_proc_pipes
        self.pipepoller.register(handle, selectors.EVENT_READ)
      File "/home/runner/micromamba/envs/cylc-functional-test/lib/python3.12/selectors.py", line 359, in register
        self._selector.register(key.fd, poller_events)
    ValueError: I/O operation on closed epoll object

and

    async def test_log_xtrigger_stdout(
        flow, scheduler, run_dir, start, log_filter
    ):
        """Output from xtriggers should appear in the scheduler log:
    
        (As per the toy example in the Cylc Docs)
        """
        # Setup a workflow:
        id_ = flow({
            'scheduler': {'allow implicit tasks': True},
            'scheduling': {
                'graph': {'R1': '@myxtrigger => foo'},
                'xtriggers': {'myxtrigger': 'myxtrigger()'}
            }
        })
        # Create an xtrigger:
        xt_lib = run_dir / id_ / 'lib/python/myxtrigger.py'
        xt_lib.parent.mkdir(parents=True, exist_ok=True)
        xt_lib.write_text(dedent(r"""
            from sys import stderr
    
            def myxtrigger():
                print('Hello World')
                print('Hello Hades', file=stderr)
                return True, {}
        """))
        schd = scheduler(id_)
        async with start(schd, level=DEBUG):
            # Set off check for x-trigger:
            task = schd.pool.get_tasks()[0]
            schd.xtrigger_mgr.call_xtriggers_async(task)
    
            # while not schd.xtrigger_mgr._get_xtrigs(task):
            while schd.proc_pool.is_not_done():
>               schd.proc_pool.process()

tests/integration/test_subprocctx.py:57: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
cylc/flow/subprocpool.py:320: in process
    self._poll_proc_pipes(proc, ctx)
cylc/flow/subprocpool.py:443: in _poll_proc_pipes
    self.pipepoller.register(handle, selectors.EVENT_READ)
../../../micromamba/envs/cylc-fast-test/lib/python3.12/selectors.py:352: in register
    key = super().register(fileobj, events, data)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <selectors.EpollSelector object at 0x7f3ba4c38200>
fileobj = <_io.FileIO [closed]>, events = 1, data = None

    def register(self, fileobj, events, data=None):
        if (not events) or (events & ~(EVENT_READ | EVENT_WRITE)):
            raise ValueError("Invalid events: {!r}".format(events))
    
        key = SelectorKey(fileobj, self._fileobj_lookup(fileobj), events, data)
    
        if key.fd in self._fd_to_key:
>           raise KeyError("{!r} (FD {}) is already registered"
                           .format(fileobj, key.fd))
E           KeyError: "<_io.FileIO name=94 mode='rb' closefd=True> (FD 94) is already registered"

../../../micromamba/envs/cylc-fast-test/lib/python3.12/selectors.py:241: KeyError