Deduplicate consecutive None items in SkipRepeatsQueue - #1348

SkipRepeatsQueue uses None to represent the absence of a previous item. As a result, q.put(None); q.put(None) queues both items and counts two unfinished tasks, although None is an immutable/hashable value and the queue promises to omit consecutive equal items.

Use a private sentinel for the empty state. Preserve the first-item short circuit for event objects, and keep delegation to queue.Queue and the existing producer lock. The regression checks queue size, task accounting, get/task_done/join, and allowing the same item again after the queue becomes empty. Parametrized controls cover zero, False and an empty string.

Validation on Windows/Python 3.12.14: before, the None regression fails and 54 queue controls pass; after, all 55 queue tests pass. An expanded run of queue, delayed-queue, observer and event tests reports 94 passed, with two existing unknown flaky-marker warnings because that optional plugin was not loaded. Ruff 0.17.2 checks/formatting, Mypy 2.4.0 for the changed source file and git diff --check pass. This is not the complete platform observer suite or eventlet/gevent compatibility validation.