force utf-8 encoding to attempt to fix Chinese

rplugin/python3/copilot.py

@@ -126,6 +126,7 @@ def ask(
                     response.status_code, f"Unknown error: {response.status_code}"
                 )
             )
+        response.encoding = "utf-8"
         for line in response.iter_lines():
             line = line.decode("utf-8").replace("data: ", "").strip()
             if line.startswith("[DONE]"):