Use pylint 2.7 for depot_tools

This includes a few fixes for specific errors, and disables several new
warnings introduced in this version, in order to allow for an incremental migration.

Bug:1262286
Change-Id: I4b8f8fc521386419a3121bbb07edc8ac83170a94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3413679
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
diff --git a/git_nav_downstream.py b/git_nav_downstream.py
index 1eeeb6a..85f9855 100755
--- a/git_nav_downstream.py
+++ b/git_nav_downstream.py
@@ -41,7 +41,8 @@
   if not downstreams:
     print("No downstream branches")
     return 1
-  elif len(downstreams) == 1:
+
+  if len(downstreams) == 1:
     run('checkout', downstreams[0], stdout=sys.stdout, stderr=sys.stderr)
   else:
     high = len(downstreams) - 1