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/sysmon/puppet_metrics.py b/scripts/sysmon/puppet_metrics.py
index 5eb90fa..9bb35cc 100644
--- a/scripts/sysmon/puppet_metrics.py
+++ b/scripts/sysmon/puppet_metrics.py
@@ -105,7 +105,7 @@
 def collect_puppet_summary():
     """Send Puppet run summary metrics."""
     try:
-        with open(LAST_RUN_FILE) as f:
+        with open(LAST_RUN_FILE, encoding="utf-8") as f:
             summary = _PuppetRunSummary(f)
     except Exception as e:
         logger.warning("Error loading Puppet run summary: %s", e)