[5.1.x] Fixed #35606, Refs #34045 -- Fixed rendering of ModelAdmin.action_checkbox for models with a __html__ method.

Thank you Claude Paroz for the report.

Regression in 85366fbca7.

Backport of 182f262b15 from main.

tests/admin_changelist/models.py

@@ -23,6 +23,12 @@ class GrandChild(models.Model):
     parent = models.ForeignKey(Child, models.SET_NULL, editable=False, null=True)
     name = models.CharField(max_length=30, blank=True)
 
+    def __str__(self):
+        return self.name
+
+    def __html__(self):
+        return f'<h2 class="main">{self.name}</h2>'
+
 
 class Genre(models.Model):
     name = models.CharField(max_length=20)