contrib: remove one test from the python 3 whitelist 439b4d005b4a introduced a use of 'hg revert --no-backup' which is currently broken. Rather than try and do a fix on the (complicated) revert code during the freeze, let's just give up on this test under Python 3 until later. Once we do fix things, the test ratchet script will re-enable it.

--- a/contrib/python3-whitelist	Mon Jul 24 13:50:25 2017 -0400
+++ b/contrib/python3-whitelist	Mon Jul 24 15:37:57 2017 -0400
@@ -24,7 +24,6 @@
 test-lrucachedict.py
 test-manifest.py
 test-merge-default.t
-test-merge-subrepos.t
 test-merge2.t
 test-merge5.t
 test-revlog-packentry.t