Add tooling that will useful in future updates, paying particular attention to difficult cases where only the upper bits on the input vary.

Use autoconfig to probe for shm_open() and shm_unlink().  Set SHM_NEEDS_LIBRT if we must
link with librt to get the shm_* functions.  Change setup.py to use the autoconfig defines.  These
changes should make it more likely that _multiprocessing/posixshmem.c gets built correctly on
different platforms.

This was a backwards incompatible change and should be clearly noted.

Related bugs:

bpo-32417: https://bugs.python.org/issue32417
bpo-35364: https://bugs.python.org/issue35364

Protect dict iterations by wrapping them with _IterationGuard in the
following methods:

- WeakValueDictionary.copy()
- WeakValueDictionary.__deepcopy__()
- WeakKeyDictionary.copy()
- WeakKeyDictionary.__deepcopy__()

Add a cell constructor, expose the cell type in the types module.

bpo-24209: In http.server script, rely on getaddrinfo to bind to preferred address based on the bind parameter. (#11767)

In http.server script, rely on getaddrinfo to bind to preferred address based on the bind parameter.

As a result, now IPv6 is used as the default (including IPv4 on dual-stack systems). Enhanced tests.

multiprocessing: provide unittests for manager classes and shareable types

* bpo-35321: Set the spec origin to frozen in frozen modules

This fix correctly sets the spec origin to
"frozen" for the _frozen_importlib module. Note that the
origin was already correctly set in _frozen_importlib_external.

* 📜🤖 Added by blurb_it.

* Make timedelta return subclass types

Previously timedelta would always return the `date` and `datetime`
types, regardless of what it is added to. This makes it return
an object of the type it was added to.

* Add tests for timedelta arithmetic on subclasses

* Make pure python timedelta return subclass types

* Add test for fromtimestamp with tz argument

* Add tests for subclass behavior in now

* Add news entry.

Fixes:
bpo-32417
bpo-35364

* More descriptive variable names in tests

Addresses Victor's comments

Python 3.8.0a1

After reading __PYVENV_LAUNCHER__ we now set sys._base_executable value for later use.
Make the same changes for macOS to avoid extra platform checks.

bpo-35692: pathlib no longer raises when checking file and directory existence on drives that are not ready (GH-11746)

Unicode version of Windows APIs are used in places, but not for GetVersionEx in Python/sysmodule.c
The wcstok_s is called on Windows in Modules/main.c and PC/launcher.c, but not in Python/pathconfig.c

Make sure that failure paths call CloseHandle outside of the function that failed

https://bugs.python.org/issue32560

LoadLibrary, GetProcAddress, FreeLibrary and GetModuleHandle acquire the system loader lock. Calling these while holding the GIL will cause a deadlock on the rare occasion that another thread is detaching and needs to destroy its thread state at the same time.

Add one char to MsiSummaryInfoGetProperty() output
Based on the patch in bpo-1104 by Anthony Tuininga (atuining) and Mark McMahon (markm).

Use correct interpretation of return value from APIs.

Added shared_memory submodule to multiprocessing in time for first alpha with cross-platform tests soon to follow.

* Fix leak for # type: ignore
* Fix the type comment leak

* Add parenthesis optional in named expressions for while statement

* Add NEWS entry