bpo-24744: Raise error in pkgutil.walk_packages if path is str

Previously an empty result list was accidentallly returned, since the
code iterated over the string as if it were the expected list of paths,
and of course found nothing.

* bpo-24484: Avoid race condition in multiprocessing cleanup

The finalizer registry can be mutated while inspected by multiprocessing
at process exit.

* Use test.support.start_threads()

* Add Misc/NEWS

Patch by Louie Lu.

When checking for reference leaks, test_c_locale_coercion is run
multiple times and so _LocaleCoercionTargetsTestCase.setUpClass() is
called multiple times. setUpClass() appends new value at each call,
so it looks like a reference leak.

Moving the setup from setUpClass() to setUpModule() avoids
this, eliminating the false alarm.

Don't run Python in an empty environment, but copy the current
environment and set PYTHONIOENCODING. So the test works also on
Python compiled in shared mode (using libpython).

* Fix race condition in signal wakeup in forkserver (followup to PR #1989)

There's an admittedly well-known race condition where ECHILD can arrive
just before the C function epoll_wait() and the latter wouldn't therefore
return EINTR.  The solution is to use set_wakeup_fd(), which was designed
to avoid such race conditions.

* Reset wakeup fd in child

Move up the discussion about 'with' keyword, so it appears earlier in the document.

* Fix bpo-30589: improve Process.exitcode with forkserver

When the child is killed, Process.exitcode should return -signum, not 255.

* Add Misc/NEWS

bpo-30624 / selectors: use bare 'except' clause instead of 'except
Exception'

If max_line_length=None is specified while using the Compat32 policy,
it is no longer ignored.

The traceback no longer displayed for SystemExit raised in a callback registered by atexit.

 import json before using json module 

Revert "bpo-29406: asyncio SSL contexts leak sockets after calling close with certain servers (#409)" (#2111)

This reverts commit a608d2d5.

The depth argument limits recursion.

SSLObject has recv() and send(), but they don't do any network io.

Also fixed a few more line endings that were missed in GH-840, which were causing failure.

- new PYTHONCOERCECLOCALE config setting
- coerces legacy C locale to C.UTF-8, C.utf8 or UTF-8 by default
- always uses C.UTF-8 on Android
- uses `surrogateescape` on stdin and stdout in the coercion
  target locales
- configure option to disable locale coercion at build time
- configure option to disable C locale warning at build time

#30624 / selectors: use bare except clause in order to not leave the fd in a bad state in case of error (#2082)

Updates checked-in line endings on several files.

This PR contains two updates to typing module:

- Support ContextManager on all versions (original PR by Jelle Zijlstra).
- Add generic AsyncContextManager.

Mock os.register_at_fork() when importing the random module, since
this function doesn't allow to unregister callbacks and so leaked
memory.

multiprocessing.Queue.get() with a timeout now polls its reader in
non-blocking mode if it succeeded to aquire the lock but the acquire
took longer than the timeout.
Co-Authored-By: Grzegorz Grzywacz <grzgrzgrz3@gmail.com>

Running our unit tests with `-bb` enabled triggered this failure.

The function '_PyArg_ParseStack()' and
'_PyArg_UnpackStack' were failing (with error
"XXX() takes Y argument (Z given)") before
the function '_PyArg_NoStackKeywords()' was called.
Thus, the latter did not raise its more meaningful
error : "XXX() takes no keyword arguments".

Mention that fnmatchcase does not call normcase, and fnmatch does.