tests: fix pyflakes warning in test-duplicateoptions.py

Differential Revision: https://phab.mercurial-scm.org/D5158

tests/test-duplicateoptions.py

@@ -10,6 +10,7 b" ignore = {b'highlight', b'win32text', b'"
 
 try:
     import sqlite3
+    del sqlite3 # unused, just checking that import works
 except ImportError:
     ignore.add(b'sqlitestore')