py: Change mp_const_* objects to macros.

py/builtintables.c

py/obj.c

py/obj.h

py/objbool.c

py/objexcept.c

py/objfun.c

py/objgenerator.c

py/objnamedtuple.c

py/objnone.c

py/objslice.c

py/objstr.c

py/objtuple.c

py/objtype.c

py/runtime.c

py/vm.c