depot_tools: Run git_footers_test on Python 3.

Bug: 1009809
Change-Id: I39bbb288a96bbb349747a3aa080f505d3b3a2cff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1835041
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
diff --git a/git_footers.py b/git_footers.py
index ad091b5..bfcf07c 100755
--- a/git_footers.py
+++ b/git_footers.py
@@ -86,7 +86,7 @@
     footer_lines = []
 
   footer_lines.reverse()
-  footers = filter(None, map(parse_footer, footer_lines))
+  footers = [footer for footer in map(parse_footer, footer_lines) if footer]
   if not footers:
     return message_lines, [], []
   if maybe_footer_lines: