Make it easier for users to make use of the backup _tkinter linked
with the third-party Tcl and Tk frameworks in /Library/Frameworks.
The two tkinter variants are now installed in separate directories
under a new lib-tkinter.  This allows per-user selection by
manipulating sys.path, directly or with PYTHONPATH.  If this
proves useful, we can supply a more convenient user interface
to supply the paths.  For now, this remains somewhat experimental.

- added test cases to ensure docstrings are reasonable
- also updates various comments in contextlib for accuracy
- identifed #19404 as an issue making it difficult to provide
  good help output on generator based context manager instances

available to compute timeout.

compute timeout.

Change Thread.join() with a negative timeout to just return.  The
behavior isn't documented then, but this restores previous
behavior.

10.8+ when using a universal Python that included a PPC architecture,
such as with a python.org 32-bit-only binary installer.

are individually controlled by #ifdef statements in the extension
code.

Issue #19375: The site module adding a "site-python" directory to sys.path, if it exists, is now deprecated.

Issue #11161: Update the documentation for ProcessPoolExecutor to note that it will not work in the interactive shell.

being defined on top of __name__.

source importlib.

unittest.TestCase in prep of running tests under frozen and source
importlib.

retry on EINTR).

of OPT for special build options.  By setting OPT, some compiler-specific
options like -fwrapv were overridden and thus not used, which could result
in broken interpreters when building with clang.

64-bit/32-bit installer for 10.6+.  It is no longer necessary
to install a third-party version of Tcl/Tk 8.5 to work around the
problems in the Apple-supplied Tcl/Tk 8.5 shipped in OS X 10.6
and later releases.

when building _tkinter.  configure has two new options; if used, both must
be specified:

  ./configure \
      --with-tcltk-includes="-I/opt/local/include" \
      --with-tcltk-libs="-L/opt/local/lib -ltcl8.5 -ltk8.5"

In addition, the options can be overridden with make:

   make \
       TCLTK_INCLUDES="-I/opt/local/include" \
       TCLTK_LIBS="-L/opt/local/lib -ltcl8.6 -ltk8.6"