Fix bytes vs str error in DownloadGerritHook.

It fails with: `write() argument must be str, not bytes`.

R=sokcevic@google.com

Change-Id: If6787140ea863ce3761a5c3747eb0b857b22fa47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3276505
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
diff --git a/git_cl.py b/git_cl.py
index fedd53e..3540382 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -3012,7 +3012,7 @@
 
   This is necessary because urllib is broken for SSL connections via a proxy.
   """
-  with open(destination, 'w') as f:
+  with open(destination, 'wb') as f:
     f.write(urllib.request.urlopen(source).read())