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>
diff --git a/git_footers.py b/git_footers.py
index 766a958..a096b6f 100755
--- a/git_footers.py
+++ b/git_footers.py
@@ -71,7 +71,8 @@
   for line in reversed(message_lines):
     if line == '' or line.isspace():
       break
-    elif parse_footer(line):
+
+    if parse_footer(line):
       footer_lines.extend(maybe_footer_lines)
       maybe_footer_lines = []
       footer_lines.append(line)
@@ -182,8 +183,8 @@
   assert len(values) <= 1, 'Multiple %s footers' % key
   if values:
     return values[0]
-  else:
-    return None
+
+  return None
 
 
 def get_position(footers):