sandbox/_mounter.py: Remove useless inheritance on `object`

In Python 3, inheriting from `object` explicitly is pointless.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

buildstream/sandbox/_mounter.py

@@ -25,7 +25,7 @@ from .. import utils, _signals
 
 
 # A class to wrap the `mount` and `umount` system commands
-class Mounter(object):
+class Mounter():
     @classmethod
     def _mount(cls, dest, src=None, mount_type=None,
                stdout=sys.stdout, stderr=sys.stderr, options=None,