Fix type of MigrationLoader.applied_migrations (#656)

Co-authored-by: Darius Marian <[email protected]>

Author: darius-blink

django-stubs/db/migrations/loader.pyi

@@ -16,7 +16,7 @@ MIGRATIONS_MODULE_NAME: str
 class MigrationLoader:
     connection: Optional[BaseDatabaseWrapper] = ...
     disk_migrations: Dict[Tuple[str, str], Migration] = ...
-    applied_migrations: Set[Tuple[str, str]] = ...
+    applied_migrations: Dict[Tuple[str, str], Migration] = ...
     ignore_no_migrations: bool = ...
     def __init__(
         self, connection: Optional[BaseDatabaseWrapper], load: bool = ..., ignore_no_migrations: bool = ...