delete Python 2 (object) compat

Bug: 302871152
Change-Id: I39636d73a6e1d69efa8ade74f75c5381651e6dc8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/390054
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
diff --git a/git_refs.py b/git_refs.py
index 1f5eaab..8e8d603 100644
--- a/git_refs.py
+++ b/git_refs.py
@@ -28,7 +28,7 @@
 R_M = "refs/remotes/m/"
 
 
-class GitRefs(object):
+class GitRefs:
     def __init__(self, gitdir):
         self._gitdir = gitdir
         self._phyref = None