In setUp() and tearDown() methods of test_concurrent_futures tests,
make sure that tests don't leak threads nor processes. Clear
explicitly the reference to the executor to make it that it's
destroyed (to prevent "dangling threads" warning).

* Revert "bpo-30854: Fix compile error when --without-threads (#2581)"

This reverts commit 0c31163.

* Revert "NEWS for 30777 (#2576)"

This reverts commit aaa917f.

* Revert "bpo-21624: IDLE -- minor htest fixes (#2575)"

This reverts commit 2000150.

* Revert "bpo-30777: IDLE: configdialog - add docstrings and improve comments (#2440)"

This reverts commit 7eb5883.

* Revert "bpo-30319: socket.close() now ignores ECONNRESET (#2565)"

This reverts commit 67e1478.

* Revert "bpo-30789: Use a single memory block for co_extra. (#2555)"

This reverts commit 378ebb6.

* Revert "bpo-30845: Enhance test_concurrent_futures cleanup (#2564)"

This reverts commit 3df9dec.

* Revert "bpo-29293: multiprocessing.Condition.notify() lacks parameter `n` (#2480)"

This reverts commit 4835041.

* Revert "Remove outdated FOX from GUI FAQ (GH-2538)"

This reverts commit d3ed287.

* Revert "bpo-6691: Pyclbr now reports nested classes and functions. (#2503)"

This reverts commit 246ff3b.

* Revert "bpo-29464: Rename METH_FASTCALL to METH_FASTCALL|METH_KEYWORDS and make (#1955)"

This reverts commit 6969eaf.

* Revert "bpo-30832: Remove own implementation for thread-local storage (#2537)"

This reverts commit aa0aa04.

* Revert "bpo-30764: Fix regrtest --fail-env-changed --forever (#2536)"

This reverts commit 5e87592.

* Revert "bpo-30822: Deduplicate ZoneInfoTest classes in test_datetime. (#2534)"

This reverts commit 34b5487.

* Revert "bpo-30822: Fix testing of datetime module. (#2530)"

This reverts commit 98b6bc3.

* bpo-30845: reap_children() now logs warnings

* bpo-30845: Enhance test_concurrent_futures cleanup

In setUp() and tearDown() methods of test_concurrent_futures tests,
make sure that tests don't leak threads nor processes. Clear
explicitly the reference to the executor to make it that it's
destroyed (to prevent "dangling threads" warning).

(cherry picked from commit 3df9dec)