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/autotest_quickmerge.py b/scripts/autotest_quickmerge.py
index af65fe9..96c9c0b 100644
--- a/scripts/autotest_quickmerge.py
+++ b/scripts/autotest_quickmerge.py
@@ -463,7 +463,7 @@
             try:
                 # Open file in a try-except block, for atomicity, instead of
                 # doing existence check.
-                with open(logfile, "r") as f:
+                with open(logfile, "r", encoding="utf-8") as f:
                     package_cp = f.readline().strip()
                     DOWNGRADE_EBUILDS.append(package_cp)
             except IOError: