Header was printed using sys.stdout, but the rest of the output was
printed using sys.stdout.buffer, causing the output to be written
before the header.

Bug: 984182
Change-Id: If6aaa05a579c3b0c73ce8ffe48f3a4e777fa16fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1790254
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>