Heavily revised, comprising revisions:
46640 - original trunk revision (backed out in r46655)
46647 - markup fix (backed out in r46655)
46692:46918 merged from branch aimacintyre-sf1454481

branch tested on buildbots (Windows buildbots had problems
not related to these changes).

46640 Patch #1454481:  Make thread stack size runtime tunable.
46647 Markup fix

The first is causing many buildbots to fail test runs, and there
are multiple causes with seemingly no immediate prospects for
repairing them.  See python-dev discussion.

Note that a branch can (and should) be created for resolving these
problems, like

svn copy svn+ssh://svn.python.org/python/trunk -r46640 svn+ssh://svn.python.org/python/branches/NEW_BRANCH

followed by merging rev 46647 to the new branch.

Use Py_UnpackTuple instead of PyArg_ParseTuple where possible.

discussion.
There are two places of documentation that still mention __context__:
Doc/lib/libstdtypes.tex -- I wasn't quite sure how to rewrite that without
spending a whole lot of time thinking about it; and whatsnew, which Andrew
usually likes to change himself.

Anyway, this is the changes to the with-statement
so that __exit__ must return a true value in order
for a pending exception to be ignored.
The PEP (343) is already updated.