Allow Final assignments in stubs (#5490)

## Summary

This fixes one incompatibility with `flake8-pyi`, and gives us a clean
pass on `typeshed`.

Author: charliermarsh

crates/ruff/resources/test/fixtures/flake8_pyi/PYI015.py

@@ -91,3 +91,4 @@ class Class1:
 field28 = builtins.str
 field29 = str
 field30 = str | bytes | None
+field31: typing.Final = field30

crates/ruff/resources/test/fixtures/flake8_pyi/PYI015.pyi

@@ -98,3 +98,4 @@ field27 = list[str]
 field28 = builtins.str
 field29 = str
 field30 = str | bytes | None
+field31: typing.Final = field30

crates/ruff/src/rules/flake8_pyi/rules/simple_defaults.rs

@@ -298,6 +298,16 @@ fn is_special_assignment(target: &Expr, semantic: &SemanticModel) -> bool {
     }
 }
 
+/// Returns `true` if this is an assignment to a simple `Final`-annotated variable.
+fn is_final_assignment(annotation: &Expr, value: &Expr, semantic: &SemanticModel) -> bool {
+    if matches!(value, Expr::Name(_) | Expr::Attribute(_)) {
+        if semantic.match_typing_expr(annotation, "Final") {
+            return true;
+        }
+    }
+    false
+}
+
 /// Returns `true` if the a class is an enum, based on its base classes.
 fn is_enum(bases: &[Expr], semantic: &SemanticModel) -> bool {
     return bases.iter().any(|expr| {
@@ -438,6 +448,9 @@ pub(crate) fn annotated_assignment_default_in_stub(
     if is_type_var_like_call(value, checker.semantic()) {
         return;
     }
+    if is_final_assignment(annotation, value, checker.semantic()) {
+        return;
+    }
     if is_valid_default_value_with_annotation(value, true, checker.locator, checker.semantic()) {
         return;
     }