[subprocess2] Replace VOID with DEVNULL

subprocess.DEVNULL was introduced in Python3 to serve same purpose
as subprocess2.VOID, so rename VOID to DEVNULL in subprocess2.

Change-Id: I6dade3306ffc3bc2441ac6083f362b099c2427e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2587758
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
diff --git a/git_cl.py b/git_cl.py
index c6fdd98..a5143d8 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -195,7 +195,7 @@
 def RunGitWithCode(args, suppress_stderr=False):
   """Returns return code and stdout."""
   if suppress_stderr:
-    stderr = subprocess2.VOID
+    stderr = subprocess2.DEVNULL
   else:
     stderr = sys.stderr
   try: