Revert "Use pylint 2.7 for depot_tools"

This reverts commit 22bf605bb63f8ff01aa7c1a2841718e3489b43d6.

Reason for revert: breaks gclient sync

Original change's description:
> 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: Ie97d686748c9c952e87718a65f401c5f6f80a5c9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3400616
> Reviewed-by: Gavin Mak <gavinmak@google.com>
> Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>

Bug: 1262286
Change-Id: Ieb946073c7886c7bf056ce843a5a48e82becf7a5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3413672
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
diff --git a/git_nav_downstream.py b/git_nav_downstream.py
index 85f9855..1eeeb6a 100755
--- a/git_nav_downstream.py
+++ b/git_nav_downstream.py
@@ -41,8 +41,7 @@
   if not downstreams:
     print("No downstream branches")
     return 1
-
-  if len(downstreams) == 1:
+  elif len(downstreams) == 1:
     run('checkout', downstreams[0], stdout=sys.stdout, stderr=sys.stderr)
   else:
     high = len(downstreams) - 1