Fixed #373 -- Added CompositePrimaryKey.

Thanks Lily Foote and Simon Charette for reviews and mentoring
this Google Summer of Code 2024 project.

Co-authored-by: Simon Charette <charette.s@gmail.com>
Co-authored-by: Lily Foote <code@lilyf.org>

tests/migrations/test_writer.py

@@ -1138,3 +1138,22 @@ class WriterTests(SimpleTestCase):
             ValueError, "'TestModel1' must inherit from 'BaseSerializer'."
         ):
             MigrationWriter.register_serializer(complex, TestModel1)
+
+    def test_composite_pk_import(self):
+        migration = type(
+            "Migration",
+            (migrations.Migration,),
+            {
+                "operations": [
+                    migrations.AddField(
+                        "foo",
+                        "bar",
+                        models.CompositePrimaryKey("foo_id", "bar_id"),
+                    ),
+                ],
+            },
+        )
+        writer = MigrationWriter(migration)
+        output = writer.as_string()
+        self.assertEqual(output.count("import"), 1)
+        self.assertIn("from django.db import migrations, models", output)