gh-83055: Reserve freed asyncio.Queue slots for woken putters - #160967

When the queue is full, get_nowait() pops a waiting putter and wakes it, but the freed slot stays open until that putter resumes. A later put() (fast path) or put_nowait() can take it; the woken putter then finds the queue full and is re-appended to the end of _putters, so items are put out of order and a waiter can be starved.

This PR reserves the freed slot for the woken putter:

• get_nowait() increments _reserved when it wakes a putter; full() counts reserved slots (qsize() + _reserved >= maxsize).
• The woken putter releases its reservation right before put_nowait(); if it is cancelled after being woken it passes the reservation to the next putter.
• shutdown() clears the counter.

This is the same idea asyncio.Semaphore uses for fairness (taking the value on the waiter's behalf). Compared to the earlier attempt (gh-112501):

• no new exceptions; full() is False still guarantees put_nowait() succeeds;
• O(1), no scanning of done futures;
• unreserved free slots remain usable, so Serhiy's example (two get_nowait() with one waiting putter, then put_nowait()) succeeds.

Behaviour change: full() may be True while qsize() < maxsize, for as long as a woken putter has not resumed. Documented in full()/put().

Getters are left as is: the issue is about the order items enter the queue, and reserving items would change the meaning of empty().

• Issue: asyncio.Queue: putting items out of order when it is full #83055

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