Set utf-8 as an encoding for open()

Not every system uses 'utf-8' as a default encoding for opening files in Python.

Before you say that there must be a way to set default encoding in one line, no, there is not. At least, I didn't found a way to do this.

tests/artifacts/generate_stix.py

@@ -41,7 +41,7 @@ def generate_test_stix_file(file_path):
         res.append(Relationship(i, "indicates", malware))
 
     bundle = Bundle(objects=res)
-    with open(file_path, "w+") as f:
+    with open(file_path, "w+", encoding="utf-8") as f:
         f.write(bundle.serialize(pretty=True))