fix appharness app_source typing (#3777)

Author: benedikt-bartscher

reflex/testing.py

@@ -115,7 +115,7 @@ class AppHarness:
 
     app_name: str
     app_source: Optional[
-        types.FunctionType | types.ModuleType | str | functools.partial
+        types.FunctionType | types.ModuleType | str | functools.partial[Any]
     ]
     app_path: pathlib.Path
     app_module_path: pathlib.Path
@@ -134,7 +134,9 @@ class AppHarness:
     def create(
         cls,
         root: pathlib.Path,
-        app_source: Optional[types.FunctionType | types.ModuleType | str] = None,
+        app_source: Optional[
+            types.FunctionType | types.ModuleType | str | functools.partial[Any]
+        ] = None,
         app_name: Optional[str] = None,
     ) -> "AppHarness":
         """Create an AppHarness instance at root.