[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

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(