depot_tools: Make git_cache Python 3 compatible.

Bug: 984182
Change-Id: Ia413435af80d03dcfa01bb2ab7480cd6a37b4161
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1717496
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
diff --git a/gclient_utils.py b/gclient_utils.py
index 78c08b1..4653953 100644
--- a/gclient_utils.py
+++ b/gclient_utils.py
@@ -597,6 +597,7 @@
         if len(in_line):
           filter_fn(in_line)
       rv = kid.wait()
+      kid.stdout.close()
 
       # Don't put this in a 'finally,' since the child may still run if we get
       # an exception.