Removed threading fallback imports.

Django imports threading in many other places without fallback.

tests/file_storage/tests.py

@@ -6,16 +6,12 @@ import os
 import shutil
 import sys
 import tempfile
+import threading
 import time
 import unittest
 import warnings
 from datetime import datetime, timedelta
 
-try:
-    import threading
-except ImportError:
-    import dummy_threading as threading
-
 from django.core.cache import cache
 from django.core.exceptions import SuspiciousOperation, SuspiciousFileOperation
 from django.core.files.base import File, ContentFile