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/diff_license_html.py b/scripts/diff_license_html.py
index 7da574a..e641a14 100644
--- a/scripts/diff_license_html.py
+++ b/scripts/diff_license_html.py
@@ -39,7 +39,7 @@
     license_rgx5 = re.compile(
         r'class="title">(Custom License .+)</a>', re.IGNORECASE
     )
-    with open(html_file, "r") as f:
+    with open(html_file, "r", encoding="utf-8") as f:
         for line in f:
             # Grep and turn
             # <span class="title">ath6k-34</span>