Issue 37069: regrtest: log unraisable exceptions and uncaught thread exceptions

Python 3.8 got 2 new hooks: sys.unraisablehook and threading.excepthook. It would be interesting to catch these exceptions and display them again in the test summary.

I modified the io.IOBase destructor to log close() exception. There are still open issues: see bpo-18748. Once these issues will be fixed, maybe it would even be interesting to make a test fail if at least one uncaught thread exception is raised or an unraisable exception is logged?

References:

* bpo-36829: sys.unraisablehook()
* bpo-1230540: threading.excepthook()

I wrote a minimum implementation: PR 13759 marks a test as "altered the execution environment" (ENV_CHANGED) if sys.unraisablehook() is called at least once. This this PR, 9 tests fail:

* test_urllib: see bpo-36918
* test_io: see bpo-18748

* test_thread: test_save_exception_state_on_error()
* test_yield_from: test_broken_getattr_handling()
* test_coroutines: test_fatal_coro_warning()
* test_cprofile: test_bad_counter_during_dealloc()
* test_raise: test_3611()
* test_ssl: test_sni_callback_raising() and test_sni_callback_wrong_return_type()

* test_generators: I'm not sure, it's maybe caused by doctests? multiple tests?

New changeset 0025350294959594e7f57aef4fc9579c77a0ed1c by Victor Stinner in branch 'master':
bpo-37069: tests use catch_unraisable_exception() (GH-13762)
https://github.com/python/cpython/commit/0025350294959594e7f57aef4fc9579c77a0ed1c

New changeset 95f61c8b1619e736bd5e29a0da0183234634b6e8 by Victor Stinner in branch 'master':
bpo-37069: regrtest uses sys.unraisablehook (GH-13759)
https://github.com/python/cpython/commit/95f61c8b1619e736bd5e29a0da0183234634b6e8

regrtest now uses :func:`sys.unraisablehook` to mark a test as "environment altered" (ENV_CHANGED) if it emits an "unraisable exception".

stage: patch review -> resolved