Support testing with Django 5.1.

Author: geoffrey-eisenbarth

tests/models.py

@@ -1,3 +1,4 @@
+import django
 from django.db import models
 
 from postgres_copy import CopyManager, CopyMapping
@@ -17,7 +18,13 @@ class MockObject(models.Model):
     class Meta:
         app_label = "tests"
         unique_together = ("name", "number")
-        index_together = ("name", "number")
+
+    def __init__(self, *args, **kwargs):
+      super().__init__(*args, **kwargs)
+      if django.get_version() <= '5.1':
+        self._meta.index_together = ("name", "number")
+      else:
+        self._meta.indexes = [models.Index(fields=["name", "number"])]
 
     def copy_name_template(self):
         return 'upper("%(name)s")'