py3: use absolute_import in import-checker

contrib/import-checker.py

@@ -1,5 +1,7 b''
 #!/usr/bin/env python
 
+from __future__ import absolute_import
+
 import ast
 import collections
 import os

tests/test-check-py3-compat.t

@@ -3,7 +3,6 b''
   $ cd "$TESTDIR"/..
 
   $ hg files 'set:(**.py)' | sed 's|\\|/|g' | xargs python contrib/check-py3-compat.py
-  contrib/import-checker.py not using absolute_import
   contrib/import-checker.py requires print_function
   doc/check-seclevel.py not using absolute_import
   doc/gendoc.py not using absolute_import