Add missing Django 5.1 classifier.

Author: fabiocaccamo

pyproject.toml

@@ -26,6 +26,7 @@ classifiers = [
     "Framework :: Django :: 4.1",
     "Framework :: Django :: 4.2",
     "Framework :: Django :: 5.0",
+    "Framework :: Django :: 5.1",
     "Intended Audience :: Developers",
     "License :: OSI Approved :: MIT License",
     "Natural Language :: English",