Also, give a more helpful error message when opened with an invalid mode string.

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.)

Reedy.

where relevant, instead send method.

can't be called reliably from a worker thread.

Patch by Guilherme Polo and Roger Serwy.

This dict entry is added a few lines after if the bzip2 module is
available, but removing this line was forgotten.

module, like hashlib.  (Patch by Roger Serwy)

multiprocessing.Connection socket can't be bound.

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

Closes #13402.

People searcing for the way to get a "service pack" will never find that we
provide it here, and people that find this function won't know what CSD is
until they run the function. On top of this, they won't know what the value
means unless they really have a service pack installed.

CSD, or Customer Service Diagnostics, is apparently no longer used, and was
rarely used term at that. Most references to it online are from
universities making Windows 2000 and XP service packs available to students.

Closes #13676.

to PyShell.PyShell.transfer_path() and changing elsewhere as needed.
Original patches by Marco Scataglini and Roger Serwy.

being read from the header vs the values being used by fseek and ftell
(Py_ssize_t for those) and how they are computed.  Py_ssize_t is used for
actual file offsets so that files greater than 2gigs could be supported.
Updates the Py_BuildValue format string to match (including several existing
wrong 'i's that should have been 'l's).