Move last sync point to its own file.
Since CPython has moved to github, I had to guestimate the point based on the last backport @rbtcollins did as the git mirror he was using evidently had different commit hashes:
```
$ git log --grep "Issue #26323"
commit 2c2a4e63d794eb55e9163322ea11b9765e9e0db5
Author: Victor Stinner <[email protected]>
Date: Fri Mar 11 22:17:48 2016 +0100
Add Mock.assert_called()
Issue #26323: Add assert_called() and assert_called_once() methods to
unittest.mock.Mock.
```2 files changed
