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Not a big deal, but the change produces compiler warnings with GCC 6.1.1:
/home/proj/python/cpython/Objects/bytesobject.c: In function ‘bytes_subscript’:
/home/proj/python/cpython/Objects/bytesobject.c:1701:13: warning: ‘slicelength’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 for (cur = start, i = 0; i < slicelength;
 ^~~
/home/proj/python/cpython/Objects/listobject.c: In function ‘list_ass_subscript’:
/home/proj/python/cpython/Objects/listobject.c:2602:13: warning: ‘slicelength’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 for (i = 0; i < slicelength; i++) {
 ^~~
/home/proj/python/cpython/Objects/unicodeobject.c: In function ‘unicode_subscript’:
/home/proj/python/cpython/Objects/unicodeobject.c:14013:16: warning: ‘slicelength’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 result = PyUnicode_New(slicelength, max_char);
 ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/media/disk/home/proj/python/cpython/Modules/_elementtree.c: In function ‘element_ass_subscr’:
/media/disk/home/proj/python/cpython/Modules/_elementtree.c:1896:50: warning: ‘slicelen’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 self->extra->children[i + newlen - slicelen] = self->extra->children[i];
 ~~~~~~~~~~~^~~~~~~~~~
/media/disk/home/proj/python/cpython/Modules/_ctypes/_ctypes.c: In function ‘Array_subscript’:
/media/disk/home/proj/python/cpython/Modules/_ctypes/_ctypes.c:4327:16: warning: ‘slicelen’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 np = PyUnicode_FromWideChar(dest, slicelen);
 ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
My build used to be free of warnings. This warning is enabled via -Wall. The reason is probably that the new macro skips the slicelength assignment if PySlice_Unpack() fails. Workarounds could be to assign or initialize slicelength to zero (at the call sites or inside the macro), or to compile with -Wno-maybe-uninitialized.