tests: define class in modern style to avoid check-code.py error

This is a part of preparation to apply checking with check-code.py on
code fragments embedded in *.t test scripts.

tests/test-hook.t

@@ -855,7 +855,7 b' preoutgoing hook can prevent outgoing ch'
   > def printtags(ui, repo, **args):
   >     ui.write(b'[%s]\n' % b', '.join(sorted(repo.tags())))
   > 
-  > class container:
+  > class container(object):
   >     unreachable = 1
   > EOF