Closes #9750

The code used to call os.path.join to build a regex but without escaping
the backslash, which lead to test failures on Windows.  Antoine Pitrou
fixed it in 557a973709de by enhancing the code to accept both / and \,
with proper escaping, but in my opinion this goes against the distutils
feature freeze, hence this change.

has left two versions of customize_compiler, the original in
distutils.sysconfig and another copy in distutils.ccompiler, with some
parts of distutils calling one and others using the other.
Complete the revert back to only having one in distutils.sysconfig as
is the case in 3.x.

Distutils-based packages with C extension modules may fail because
Apple has removed gcc-4.2, the version used to build python.org
64-bit/32-bit Pythons.  If the user does not explicitly override
the default C compiler by setting the CC environment variable,
Distutils will now attempt to compile extension modules with clang
if gcc-4.2 is required but not found. Also as a convenience, if
the user does explicitly set CC, substitute its value as the default
compiler in the Distutils LDSHARED configuration variable for OS X.
(Note, the python.org 32-bit-only Pythons use gcc-4.0 and the 10.4u
SDK, neither of which are available in Xcode 4.  This change does not
attempt to override settings to support their use with Xcode 4.)

Reported by Miki Tebeka.

where relevant, instead send method. Patch contributed by Brian Brazil.

can't be called reliably from a worker thread.

Patch by Guilherme Polo and Roger Serwy.

module, like hashlib.  (Patch by Roger Serwy)

multiprocessing.Connection socket can't be bound.

Closes #13402.

a random deadlock when fork() is called in a multithreaded process in debug
mode, and make PyOS_AfterFork() more robust.

Fedora buildbot.

Closes #13676.