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.
--- a/tests/test-hook.t Mon Feb 18 00:27:25 2019 +0900
+++ b/tests/test-hook.t Mon Feb 18 00:27:25 2019 +0900
@@ -855,7 +855,7 @@
> def printtags(ui, repo, **args):
> ui.write(b'[%s]\n' % b', '.join(sorted(repo.tags())))
>
- > class container:
+ > class container(object):
> unreachable = 1
> EOF