[3.13] gh-127906: Add missing sys import to test_cppext (GH-132902) (#132904)

gh-127906: Add missing sys import to test_cppext (GH-132902)
(cherry picked from commit 9cba148)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: miss-islington

Lib/test/test_cppext/setup.py

@@ -3,6 +3,7 @@
 import os
 import platform
 import shlex
+import sys
 import sysconfig
 from test import support