Fix SF buf #480096: Assign to __debug__ still allowed

Easy enough to catch assignment in the compiler.  The perverse user
can still change the value of __debug__, but that may be the least he
can do.
diff --git a/Python/compile.c b/Python/compile.c
index c8a5669..1a46064 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -5459,8 +5459,13 @@
 			n = CHILD(n, 1);
 			goto loop;
 		} else if (TYPE(tmp) == NAME) {
-			if (strcmp(STR(tmp), "__debug__") == 0)
-				symtable_warn(st, ASSIGN_DEBUG);
+			if (strcmp(STR(tmp), "__debug__") == 0) {
+				PyErr_SetString(PyExc_SyntaxError, 
+						ASSIGN_DEBUG);
+				PyErr_SyntaxLocation(st->st_filename,
+					     st->st_cur->ste_opt_lineno);
+				st->st_errors++;
+			}
 			symtable_add_def(st, STR(tmp), DEF_LOCAL | def_flag);
 		}
 		return;