added a lock on a critical path when regenerating pages. this should allow all images to load correctly

comic/tests/test_models.py

@@ -304,4 +304,4 @@ class ComicBookTests(TestCase):
 
         c.login(username="test", password="test")
         response = c.get(f"/comic/read/{urlsafe_base64_encode(book.selector.bytes)}/0/img")
-        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.status_code, 200)