…two threads attempt to commit the last pending removal (pythonGH-27921)

Fixes:
Traceback (most recent call last):
  File "/home/graingert/projects/asyncio-demo/demo.py", line 36, in <module>
    sys.exit(main())
  File "/home/graingert/projects/asyncio-demo/demo.py", line 30, in main
    test_all_tasks_threading()
  File "/home/graingert/projects/asyncio-demo/demo.py", line 24, in test_all_tasks_threading
    results.append(f.result())
  File "/usr/lib/python3.10/concurrent/futures/_base.py", line 438, in result
    return self.__get_result()
  File "/usr/lib/python3.10/concurrent/futures/_base.py", line 390, in __get_result
    raise self._exception
  File "/usr/lib/python3.10/concurrent/futures/thread.py", line 52, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/lib/python3.10/asyncio/runners.py", line 47, in run
    _cancel_all_tasks(loop)
  File "/usr/lib/python3.10/asyncio/runners.py", line 56, in _cancel_all_tasks
    to_cancel = tasks.all_tasks(loop)
  File "/usr/lib/python3.10/asyncio/tasks.py", line 53, in all_tasks
    tasks = list(_all_tasks)
  File "/usr/lib/python3.10/_weakrefset.py", line 60, in __iter__
    with _IterationGuard(self):
  File "/usr/lib/python3.10/_weakrefset.py", line 33, in __exit__
    w._commit_removals()
  File "/usr/lib/python3.10/_weakrefset.py", line 57, in _commit_removals
    discard(l.pop())
IndexError: pop from empty list

Also fixes:
Exception ignored in: weakref callback <function WeakKeyDictionary.__init__.<locals>.remove at 0x00007fe82245d2e0>
Traceback (most recent call last):
  File "/usr/lib/pypy3/lib-python/3/weakref.py", line 390, in remove
    del self.data[k]
KeyError: <weakref at 0x00007fe76e8d8180; dead>
Exception ignored in: weakref callback <function WeakKeyDictionary.__init__.<locals>.remove at 0x00007fe82245d2e0>
Traceback (most recent call last):
  File "/usr/lib/pypy3/lib-python/3/weakref.py", line 390, in remove
    del self.data[k]
KeyError: <weakref at 0x00007fe76e8d81a0; dead>
Exception ignored in: weakref callback <function WeakKeyDictionary.__init__.<locals>.remove at 0x00007fe82245d2e0>
Traceback (most recent call last):
  File "/usr/lib/pypy3/lib-python/3/weakref.py", line 390, in remove
    del self.data[k]
KeyError: <weakref at 0x000056548f1e24a0; dead>

See: https://github.com/agronholm/anyio/issues/362GH-issuecomment-904424310
See also: https://bugs.python.org/issue29519

Co-authored-by: Łukasz Langa <lukasz@langa.pl>
(cherry picked from commit 206b21e)

Co-authored-by: Thomas Grainger <tagrain@gmail.com>