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.

Previously a Python 2.x compatible hack was used for
multiprocessing.sharedctypes.Array().  Also the documented
signature was wrong.

Proxy classes in multiprocessing do not need these methods in Python 3.x.

Patch by Sidney San Martín.

Add attrib keyword to Element and SubElement in _elementtree.
Patch developed with Ezio Melotti.

Issue #14775: Fix a potential quadratic dict build-up due to the garbage collector repeatedly trying to untrack dicts.
Additional comments by Tim Silk.

Issue #14775: Fix a potential quadratic dict build-up due to the garbage collector repeatedly trying to untrack dicts.
Additional comments by Tim Silk.

An issue in ctypes.c_longdouble, ctypes.c_double, and ctypes.c_float that
caused an incorrect exception to be returned in the case of overflow has been
fixed.

An issue in ctypes.c_longdouble, ctypes.c_double, and ctypes.c_float that
caused an incorrect exception to be returned in the case of overflow has been
fixed.

Added python3 symlink; removed Distribute-related code, docs and comments; changed Mac OS X computation to determine framework builds.