bisect-kit: fix pylint warnings

Follow chromite's new pylint rules. This CL fixes the trivial ones. More
fixes will come later.

BUG=None
TEST=cros lint

Change-Id: I18bfce6f8cb43bf29ebc3d5af1c5604ecdf50d49
Reviewed-on: https://chromium-review.googlesource.com/1772797
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Commit-Ready: Kuang-che Wu <kcwu@chromium.org>
Commit-Ready: Zheng-Jie Chang <zjchang@google.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Zheng-Jie Chang <zjchang@google.com>
diff --git a/bisect_kit/git_util.py b/bisect_kit/git_util.py
index 4844dec..59d9565 100644
--- a/bisect_kit/git_util.py
+++ b/bisect_kit/git_util.py
@@ -454,7 +454,7 @@
     result.append((int(commit_time), git_rev))
 
   if padding:
-    assert before or after, "padding=True make no sense if they are both None"
+    assert before or after, 'padding=True make no sense if they are both None'
     if before is not None and get_rev_by_time(
         git_repo, before, branch, path=path):
       before = int(before)