Fix django deprecation warning for django.utils.timezone.utc (#48)

The ImportError was always thrown:
No module named 'datetime.timezone'; 'datetime' is not a package

Author: dzonecat

ninja_jwt/utils.py

@@ -11,9 +11,9 @@
 from ninja_jwt import exceptions
 
 try:
-    from datetime.timezone import utc
+    from datetime import timezone
 except ImportError:
-    from django.utils.timezone import utc
+    from django.utils import timezone
 
 
 logger = logging.getLogger("django")
@@ -45,7 +45,7 @@ def import_callable(path_or_callable):
 
 def make_utc(dt):
     if settings.USE_TZ and is_naive(dt):
-        return make_aware(dt, timezone=utc)
+        return make_aware(dt, timezone=timezone.utc)
 
     return dt