Issue #14963: Added test cases for contextlib.ExitStack exception handling behaviour (Initial patch by Alon Horev)

PyObject_GC_Resize() with incorrect error handling.  In case of errors,
the original object would be leaked.  This checkin fixes those cases.

Or perhaps more hackish, depending on your perspective.  But at least this
way it is now possible to run the individual tests using the unittest CLI.

_elementtree, with a test.

Close #14947: add missing cross-reference to Language Definition from the new dynamic type creation functions. Also cleaned up the general wording of the docs

subprocess and trace modules.

byte literals supported in Python 3.3.

Patch by Oleg Plakhotnyuk.

This feature was supposed to be part of the initial email6 checkin, but it got
lost in my big refactoring.

In this patch I'm not providing an easy way to turn off the errors, but they
only happen when a header is added programmatically, and it is almost never
the right thing to do to allow the duplicate to be added.  An application that
needs to add duplicates of unique headers can create a policy subclass to
allow it.

 * Formatting string, int, float and complex use the _PyUnicodeWriter API. It
   avoids a temporary buffer in most cases.
 * Add _PyUnicodeWriter_WriteStr() to restore the PyAccu optimization: just
   keep a reference to the string if the output is only composed of one string
 * Disable overallocation when formatting the last argument of str%args and
   str.format(args)
 * Overallocation allocates at least 100 characters: add min_length attribute
   to the _PyUnicodeWriter structure
 * Add new private functions: _PyUnicode_FastCopyCharacters(),
   _PyUnicode_FastFill() and _PyUnicode_FromASCII()

The speed up is around 20% in average.