test in Django 5.1 pre-release

Author: PetrDlouhy

Diff for: tox.ini

@@ -7,7 +7,7 @@ envlist =
     py{37,38,39,310}-django{32}
     py{38,39,310}-django{40}
     py{38,39,310,311}-django{41}
-    py{310,311,312}-django{42,50}
+    py{310,311,312}-django{42,50,51}
     coverage-report
 
 [gh-actions]
@@ -32,6 +32,7 @@ deps=
     django41: Django>=4.1,<4.2
     django42: Django>=4.2,<5.0
     django50: Django>=5.0,<5.1
+    django51: git+https://github.com/django/django@main  # TODO: update when Django 5.1 is released
     coverage[toml]
     pillow
     ipdb