Antoine Pitrou found a variation that worked for him on the
thread+fork tests, and added an important

    self._is_stopped = True

to the after-fork code.  I confess I don't know why things passed
before.  But then mixing fork with threads is insane ;-)

Unixy buildbots were failing the thread + fork tests :-(

Documented some obscurities, and assert'ed ._stop()'s crucial precondition.

The fix for issue 18808 left us checking two things to be sure a Thread
was done:  an Event (._stopped) and a mutex (._tstate_lock).  Clumsy &
brittle.  This patch removes the Event, leaving just a happy lock :-)

The bulk of the patch removes two excruciating tests, which were
verifying sanity of the internals of the ._stopped Event after a fork.
Thanks to Antoine Pitrou for verifying that's the only real value
these tests had.

One consequence of moving from an Event to a mutex:  waiters (threads
calling Thread.join()) used to block each on their own unique mutex
(internal to the ._stopped event), but now all contend on the same
mutex (._tstate_lock).  These approaches have different performance
characteristics on different platforms.  I don't think it matters in
this context.

module is present (to record skipped tests)

(AFAICT, in theory, we must reset all the locks, not just those in use)

'--timeout' option requires faulthandler.dump_traceback_later).

faulthandler module if the variable is non-empty. Same behaviour than other
variables like PYTHONDONTWRITEBYTECODE.

default RLIMIT_NOFILE hard limit can be RLIMIT_INFINITY.

to avoid one extra FD per Connection.

Fix the dead link of IEEE_854-1987 standard with the Wikipedia entry.
Addresses issue #18438

Addresses issue #18438

The setobject freelist was consuming memory but not providing much value.
Even when a freelisted setobject was available, most of the setobject
fields still needed to be initialized and the small table still required
a memset().  This meant that the custom freelisting scheme for sets was
providing almost no incremental benefit over the default Python freelist
scheme used by _PyObject_Malloc() in Objects/obmalloc.c.

test_is_alive_after_fork is failing on some old Linux kernels, but
passing on all newer ones.  Since virtually anything can go wrong
with locks when mixing threads with fork, replace the most likely
cause with a redundant simple data member.

When test.support was converted to a package, it started silently
skipping the tests which needed to download support data to run.

This change refactors the affected code, and also tidies up
test.support.findfile to remove the unused *here* parameter, document
the *subdir* parameter and rename the *filename* parameter to avoid
shadowing the file builtin and be consistent with the documentation.

The unexpected skips were noticed and reported by Zachary Ware

Correct Profile class usage example. Addresses issue #18033.
Patch contributed by Olivier Hervieu and Dmi Baranov.

Patch contributed by Olivier Hervieu and Dmi Baranov.