[FIX] oca-gen-addon-readme: add missing utf-8 encoding

tools/gen_addon_readme.py

@@ -408,7 +408,7 @@ def gen_one_addon_index(readme_filename):
     index_dir = os.path.join(addon_dir, "static", "description")
     index_filename = os.path.join(index_dir, "index.html")
     if os.path.exists(index_filename):
-        with open(index_filename) as f:
+        with open(index_filename, encoding="utf-8") as f:
             if "oca-gen-addon-readme" not in f.read():
                 # index was created manually
                 return