bpo-45835: Fix race condition in test_queue (#29601)

Some of the tests in test_queue had a race condition in which a
non-sentinel value could be enqueued after the final sentinel value
leading to not all the inputs being processed (and test failures).

This changes feed() to enqueue a sentinel once the inputs are exhausted,
which guarantees that the final queued object is a sentinel. This
requires the number of feeder threads to match the number of consumer
threads, but that's already the case in the relevant tests.