add comment to test requirements

Co-authored-by: Bruno Alla <browniebroke@users.noreply.github.com>

Author: kulikjak

requirements/requirements-testing.txt

@@ -5,4 +5,4 @@ pytest-django>=4.5.2,<5.0
 importlib-metadata<5.0
 # temporary pin of attrs
 attrs==22.1.0
-pytz
+pytz  # Remove when dropping support for Django<5.0