`pytest`'s`LogCaptureHandler` prevents garbage collection of `Worker` instances (and others)

[hendrikmakait]

In tests that raise an exception, the LogCaptureHandler from pytest prevents garbage collection by maintaining a reference to the raised exception and its traceback:

(This reference chain was created with objgraph.find_backref_chain https://mg.pov.lt/objgraph/#memory-leak-example)

Keeping those references alive prevents garbage collection of otherwise released objects and therefore testing for proper garbage collection of Scheduler, TaskState and Worker instances as requested by #6250. For example, in #6344, the failures of distributed/tests/test_scheduler.py::test_rebalance_workers_and_keys and several other tests are caused by this.

One possible for the failures in #6344 would be to manually clear the handlers during check_instances(), though they seem to be hard to access, which results in a rather brittle solution:

handlers = logging.getLogger().handlers
for handler in handlers:
    if hasattr(handler, "records"):
        handler.records.clear()

[hendrikmakait]

This also prevents garbage collection of workers_state_machine.TaskState instances.

[fjetter]

This is only a problem if we're logging a record using exc_info=True or does this come up in a different context as well?