[stubtest] Respect @final runtime decorator, enforce it in stubs (#14922)

Now, runtime code like

```python
from typing import final

@Final
class A: ...
``` 

Will require `@final` decorator in stubs as well.

Closes #14921

Author: sobolevn

mypy/stubtest.py

@@ -419,6 +419,21 @@ class SubClass(runtime):  # type: ignore[misc]
         # Examples: ctypes.Array, ctypes._SimpleCData
         pass
 
+    # Runtime class might be annotated with `@final`:
+    try:
+        runtime_final = getattr(runtime, "__final__", False)
+    except Exception:
+        runtime_final = False
+
+    if runtime_final and not stub.is_final:
+        yield Error(
+            object_path,
+            "has `__final__` attribute, but isn't marked with @final in the stub",
+            stub,
+            runtime,
+            stub_desc=repr(stub),
+        )
+
 
 def _verify_metaclass(
     stub: nodes.TypeInfo, runtime: type[Any], object_path: list[str]
@@ -1339,7 +1354,7 @@ def verify_typealias(
         "__origin__",
         "__args__",
         "__orig_bases__",
-        "__final__",
+        "__final__",  # Has a specialized check
         # Consider removing __slots__?
         "__slots__",
     }

mypy/test/teststubtest.py

@@ -1139,6 +1139,45 @@ def test_not_subclassable(self) -> Iterator[Case]:
             error="CannotBeSubclassed",
         )
 
+    @collect_cases
+    def test_has_runtime_final_decorator(self) -> Iterator[Case]:
+        yield Case(
+            stub="from typing_extensions import final",
+            runtime="from typing_extensions import final",
+            error=None,
+        )
+        yield Case(
+            stub="""
+            @final
+            class A: ...
+            """,
+            runtime="""
+            @final
+            class A: ...
+            """,
+            error=None,
+        )
+        yield Case(  # Runtime can miss `@final` decorator
+            stub="""
+            @final
+            class B: ...
+            """,
+            runtime="""
+            class B: ...
+            """,
+            error=None,
+        )
+        yield Case(  # Stub cannot miss `@final` decorator
+            stub="""
+            class C: ...
+            """,
+            runtime="""
+            @final
+            class C: ...
+            """,
+            error="C",
+        )
+
     @collect_cases
     def test_name_mangling(self) -> Iterator[Case]:
         yield Case(