api: update open calls to use explicit encoding

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

Change-Id: Ifb495351ed9f12a850320306ff708a1c5a4d9692
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4290042
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Lizzy Presland <zland@google.com>
Commit-Queue: Lizzy Presland <zland@google.com>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
diff --git a/api/controller/firmware.py b/api/controller/firmware.py
index f9e7d36..122e5f7 100644
--- a/api/controller/firmware.py
+++ b/api/controller/firmware.py
@@ -58,7 +58,7 @@
         cmd += [subcmd]
 
         result = cros_build_lib.run(cmd, check=False)
-        with open(tmpfile.name, "r") as f:
+        with open(tmpfile.name, "r", encoding="utf-8") as f:
             response = f.read()
 
     if metric_proto:
@@ -198,7 +198,7 @@
         )
         file_paths = []
         if os.path.exists(metadata_path):
-            with open(metadata_path, "r") as f:
+            with open(metadata_path, "r", encoding="utf-8") as f:
                 metadata = json_format.Parse(
                     f.read(), firmware_pb2.FirmwareArtifactInfo()
                 )