* Add Py_UNREACHABLE() as an alias to abort().
* Use Py_UNREACHABLE() instead of assert(0)
* Convert more unreachable code to use Py_UNREACHABLE()
* Document Py_UNREACHABLE() and a few other macros.

Enabled by default was a temporary expedient.  The fix is to add a user override to enable.

New -n flag allow overriding localhost with custom value,
for example to run from containers.

Fix Coverity CID 1417587: _PyMem_Initialize() contains code which is
never executed.

Replace the runtime check with a build assertion.

join_thread() joins a thread but raises an AssertionError if the
thread is still alive after timeout seconds.

Call thread.join() to prevent the "dangling thread" warning.

Use _thread.count() to wait until threads exit. The new context
manager prevents the "dangling thread" warning.

Call thread.join() to prevent the "dangling thread" warning.

* Avoid calling "PyObject_GetAttrString()" (and potentially executing user code) with a live exception set.

* Ignore only AttributeError on attribute lookups in ElementTree.XMLParser() and propagate all other exceptions.

bpo-31471: Fix assertion failure in subprocess.Popen() on Windows, in case env has a bad keys() method. (#3580)

This makes the default behavior (without specifying `globalns` manually) more
predictable for users, finds the right globalns automatically.

Implementation for classes assumes has a `__module__` attribute and that module
is present in `sys.modules`.  It does this recursively for all bases in the
MRO.  For modules, the implementation just uses their `__dict__` directly.

This is backwards compatible, will just raise fewer exceptions in naive user
code.

Originally implemented and reviewed at https://github.com/python/typing/pull/470.

A bunch of code currently uses PyInterpreterState.modules directly instead of PyImport_GetModuleDict(). This complicates efforts to make changes relative to sys.modules. This patch switches to using PyImport_GetModuleDict() uniformly. Also, a number of related uses of sys.modules are updated for uniformity for the same reason.

Note that this code was already reviewed and merged as part of #1638. I reverted that and am now splitting it up into more focused parts.

* bpo-31234: Join threads in test_hashlib

Use thread.join() to wait until the parallel hash tasks complete
rather than using events. Calling thread.join() prevent "dangling
thread" warnings.

* test_hashlib: minor PEP 8 coding style fixes

Call thread.join() on threads to prevent the "dangling threads"
warning.

Some of the proxied methods use internal Logger state which isn't proxied,
causing failures if an adapter is applied to another adapter.

This commit fixes the issue, adds a new test for the use case.

Cast Py_buffer.len (Py_ssize_t, signed) to size_t (unsigned) to
prevent the following warning:

Modules/_ssl.c:3089:21: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]

Resolves bpo-31420.

(This was accidentally reverted when in #3565.)

PR #1638, for bpo-28411, causes problems in some (very) edge cases. Until that gets sorted out, we're reverting the merge. PR #3506, a fix on top of #1638, is also getting reverted.

The `blake2b` function does not take the `data` keyword argument.
The hex digest returned by sign was a string, whereas compare_digest expects bytes-like objects.
Typo fix: compare_digesty -> compare_digest

Update the link from Misc/NEWS to Misc/NEWS.d

Try to fix the "Warning -- threading_cleanup() failed to cleanup 1
threads" warning in test.lock_tests: wait a little bit longer to give
time to the threads to complete.

Warning seen on test_thread and test_importlib.

* Copy changes to typing from upstream repo
* Add NEWS entry

bpo-31418: Fix an assertion failure in PyErr_WriteUnraisable() in case of an exception with a bad __module__ attribute. (#3539)

support.threading_cleanup() waits for 1 second before emitting a
warning if there are threads running in the background. With this
change, it now emits the warning immediately, to be able to catch
bugs more easily.

Explicitly state that communicate() closes stdin after writing input to it.

Clear also self.server_thread attribute in TestTimeouts.tearDown().

Call POP3.close(), don't close close directly the sock attribute.

Explicitly clear the server attribute in test_ftplib and test_poplib
to prevent dangling thread.

socketserver.ThreadingMixIn now keeps a list of non-daemonic threads
to wait until all these threads complete in server_close().

Reenable test_logging skipped tests.

Fix SocketHandlerTest.tearDown(): close the socket handler before
stopping the server, so the server can join threads.

The ConfigSocketReceiver.serve_until_stopped() method from
logging.config.listen() now calls server_close() (of
socketserver.ThreadingTCPServer) rather than closing manually the
socket.

While this change has no effect yet, it will help to prevent dangling
threads once ThreadingTCPServer.server_close() will join spawned
threads (bpo-31233).

The last commit contained lines longer than 80 characters.

Make it clear that Ellipsis tokens also have type attribute set to token.OP.

ProcessPoolShutdownTest.test_del_shutdown() now closes the call queue
and joins its thread, to prevent leaking a dangling thread.

* test_thread.test_forkinthread() now waits until the thread completes.
* Check the status in the test method, not in the thread function
* Don't ignore RuntimeError anymore: since the commit
  346cbd35 (bpo-16500,
  os.register_at_fork(), os.fork() cannot fail anymore with
  RuntimeError.
* Replace 0.01 literal with a new POLL_SLEEP constant
* test_forkinthread(): test if os.fork() exists rather than testing
  the platform.

bpo-31428: Prevent raising a SystemError in case the memo arg of ElementTree.Element.__deepcopy__() isn't a dictionary. (#3512)