tests: update test-obsolete to pass our import checker

tests/test-obsolete.t

@@ -1207,8 +1207,9 b' Test issue 4506'
 Test heads computation on pending index changes with obsolescence markers
   $ cd ..
   $ cat >$TESTTMP/test_extension.py  << EOF
+  > from __future__ import absolute_import
+  > from mercurial.i18n import _
   > from mercurial import cmdutil, registrar
-  > from mercurial.i18n import _
   > 
   > cmdtable = {}
   > command = registrar.command(cmdtable)