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/subcommand.py b/subcommand.py
index 4087627..f77a8e4 100644
--- a/subcommand.py
+++ b/subcommand.py
@@ -147,11 +147,11 @@
         reverse=True)
     if (hamming_commands[0][0] - hamming_commands[1][0]) < 0.3:
       # Too ambiguous.
-      return
+      return None
 
     if hamming_commands[0][0] < 0.8:
       # Not similar enough. Don't be a fool and run a random command.
-      return
+      return None
 
     return commands[hamming_commands[0][1]]