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