py3: use absolute_import in mockblackbox.py

tests/mockblackbox.py

@@ -1,4 +1,7 b''
-from mercurial import util
+from __future__ import absolute_import
+from mercurial import (
+    util,
+)
 
 def makedate():
     return 0, 0

tests/test-check-py3-compat.t

@@ -42,7 +42,6 b''
   setup.py not using absolute_import
   tests/heredoctest.py requires print_function
   tests/md5sum.py not using absolute_import
-  tests/mockblackbox.py not using absolute_import
   tests/printenv.py not using absolute_import
   tests/readlink.py not using absolute_import
   tests/readlink.py requires print_function