tests: conditionalize output for Python 3

It appears that the random.randint() behavior is different between Python 2
and Python 3. So make the test conditional on that.

This makes the test pass on Python 3.7 (and presumably other Python 3 versions).

Differential Revision: https://phab.mercurial-scm.org/D6964

tests/test-upgrade-repo.t

@@ -617,7 +617,8 b' We can restrict optimization to some rev'
 Check that the repo still works fine
 
   $ hg log -G --stat
-  @  changeset:   2:76d4395f5413
+  @  changeset:   2:76d4395f5413 (no-py3 !)
+  @  changeset:   2:fca376863211 (py3 !)
   |  tag:         tip
   |  parent:      0:ba592bf28da2
   |  user:        test