Issue #17919: add missing import of USHRT_MAX

Lib/test/test_devpoll.py

@@ -4,6 +4,7 @@
 
 import os, select, random, unittest, sys
 from test.support import TESTFN, run_unittest
+from _testcapi import USHRT_MAX
 
 try:
     select.devpoll