[4.0.x] Fixed #33234 -- Fixed autodetector crash for proxy models inheriting from non-model class.

Regression in aa4acc164d.

Thanks Kevin Marsh for the report.
Backport of dab48b7482 from main

django/db/migrations/state.py

@@ -476,6 +476,8 @@ class ProjectState:
 
     def _find_concrete_model_from_proxy(self, proxy_models, model_state):
         for base in model_state.bases:
+            if not (isinstance(base, str) or issubclass(base, models.Model)):
+                continue
             base_key = make_model_tuple(base)
             base_state = proxy_models.get(base_key)
             if not base_state:

tests/migrations/test_autodetector.py

@@ -1779,6 +1779,29 @@ class AutodetectorTests(TestCase):
         self.assertOperationAttributes(changes, "testapp", 0, 0, name="AuthorProxy")
         self.assertOperationAttributes(changes, "testapp", 0, 1, name="AuthorProxy", options={})
 
+    def test_proxy_non_model_parent(self):
+        class Mixin:
+            pass
+
+        author_proxy_non_model_parent = ModelState(
+            'testapp',
+            'AuthorProxy',
+            [],
+            {'proxy': True},
+            (Mixin, 'testapp.author'),
+        )
+        changes = self.get_changes(
+            [self.author_empty],
+            [self.author_empty, author_proxy_non_model_parent],
+        )
+        self.assertNumberMigrations(changes, 'testapp', 1)
+        self.assertOperationTypes(changes, 'testapp', 0, ['CreateModel'])
+        self.assertOperationAttributes(
+            changes, 'testapp', 0, 0, name='AuthorProxy',
+            options={'proxy': True, 'indexes': [], 'constraints': []},
+            bases=(Mixin, 'testapp.author'),
+        )
+
     def test_proxy_custom_pk(self):
         """
         #23415 - The autodetector must correctly deal with custom FK on proxy