fix applied_migrations typing of recorder.pyi (#230)

The function returns a dictionary, not a set. 

Values are instances of the Migration model declared in the class - not sure if / how we can re-use that type though

django-stubs/db/migrations/recorder.pyi

@@ -15,7 +15,7 @@ class MigrationRecorder:
     def migration_qs(self) -> QuerySet[Any]: ...
     def has_table(self) -> bool: ...
     def ensure_schema(self) -> None: ...
-    def applied_migrations(self) -> set[tuple[str, str]]: ...
+    def applied_migrations(self) -> dict[tuple[str, str], Any]: ...
     def record_applied(self, app: str, name: str) -> None: ...
     def record_unapplied(self, app: str, name: str) -> None: ...
     def flush(self) -> None: ...