do not save unused variable

Constructing a PackageDefinition is enough to assert that it raises an
error.

unittests/allplatformstests.py

@@ -3598,7 +3598,7 @@ class AllPlatformTests(BasePlatformTests):
                 filename = foo/real.wrap
                 '''))
         with self.assertRaisesRegex(WrapException, 'wrap-redirect filename must be in the form foo/subprojects/bar.wrap'):
-            wrap = PackageDefinition(redirect_wrap)
+            PackageDefinition(redirect_wrap)
 
         # Correct redirect
         with open(redirect_wrap, 'w', encoding='utf-8') as f: