fix: Fix minor issue of problem admin API

backend/problem/views/admin.py

@@ -9,6 +9,7 @@
 from django.http import StreamingHttpResponse
 
 from contest.models import Contest, ContestStatus
+from course.models import Course
 from judge.dispatcher import SPJCompiler
 from submission.models import Submission
 from utils.api import APIView, CSRFExemptAPIView, validate_serializer, APIError