Bump serious_python to 4.1.0 in template

Update serious_python from 4.0.0 to 4.1.0 in sdk/python/templates/build/{{cookiecutter.out_dir}}/pubspec.yaml so generated projects use the newer dependency version.

Author: FeodorFitsner

sdk/python/templates/build/{{cookiecutter.out_dir}}/pubspec.yaml

@@ -18,7 +18,7 @@ dependencies:
   flet:
     path: ../../../../../packages/flet
 
-  serious_python: 4.0.0
+  serious_python: 4.1.0
 
   # MsgPack codec used by the dart_bridge FletBackendChannel implementation
   # in lib/main.dart — matches the wire format flet's existing socket