Write up 3.16 announcement

Author: browniebroke

docs/community/3.16-announcement.md

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+# Django REST framework 3.16
+
+At the Internet, on March 31th, 2025, we are happy to announce the release of Django REST framework 3.16.
+
+## Updated Django and Python support
+
+The latest release now fully supports Django 5.1 and the upcoming 5.2 LTS as well as Python 3.13.
+
+The current minimum versions of Django still is now 4.2 and Python 3.9.
+
+## Django LoginRequiredMiddleware
+
+The new `LoginRequiredMiddleware` introduced by Django 5.1 can now be used alongside Django REST Framework, however it is not honored for API views as an equivalent behaviour can be configured via `DEFAULT_AUTHENTICATION_CLASSES`.
+
+## Improved support for UniqueConstraint
+
+The generation of validators for [UniqueConstraint](https://docs.djangoproject.com/en/stable/ref/models/constraints/#uniqueconstraint) has been improved to support better nullable fields and constraints with conditions.
+
+## Other fixes and improvements
+
+There are a number of fixes and minor improvements in this release, ranging from documentation, internal infrastructure (typing, testing, requirements, deprecation, etc.), security and overall behaviour.
+
+See the [release notes](release-notes.md) page for a complete listing.