scripts: update open calls to use explicit encoding

BUG=b:187789896
TEST=`cros lint` is clean here

Change-Id: Ia54475869c17292505537334362cd4cff6438b14
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4290041
Commit-Queue: Cindy Lin <xcl@google.com>
Reviewed-by: Cindy Lin <xcl@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/gen_luci_scheduler.py b/scripts/gen_luci_scheduler.py
index 788876a..982b1df 100644
--- a/scripts/gen_luci_scheduler.py
+++ b/scripts/gen_luci_scheduler.py
@@ -251,6 +251,8 @@
     branch_config = chromeos_config.BranchScheduleConfig()
 
     with (
-        open(options.file_out, "w") if options.file_out else sys.stdout
+        open(options.file_out, "w", encoding="utf-8")
+        if options.file_out
+        else sys.stdout
     ) as fh:
         fh.write(genLuciSchedulerConfig(site_config, branch_config))