threading: fix ExceptHookArgs being a function instead of a type (#4768)

The previous typing meant `threading.ExceptHookArgs` could not be used
to type a value.

The new typing follows what cpython does in the happy path (`_thread`
exists rather than the pure-python fallback being used).

Fixes #4767.

stdlib/3/_thread.pyi

@@ -1,7 +1,7 @@
 import sys
 from threading import Thread
 from types import TracebackType
-from typing import Any, Callable, Dict, NamedTuple, NoReturn, Optional, Tuple, Type
+from typing import Any, Callable, Dict, NoReturn, Optional, Tuple, Type
 
 error = RuntimeError
 
@@ -29,10 +29,13 @@ TIMEOUT_MAX: float
 
 if sys.version_info >= (3, 8):
     def get_native_id() -> int: ...  # only available on some platforms
-    class ExceptHookArgs(NamedTuple):
-        exc_type: Type[BaseException]
-        exc_value: Optional[BaseException]
-        exc_traceback: Optional[TracebackType]
-        thread: Optional[Thread]
-    def _ExceptHookArgs(args: Any) -> ExceptHookArgs: ...
-    _excepthook: Callable[[ExceptHookArgs], Any]
+    class _ExceptHookArgs(Tuple[Type[BaseException], Optional[BaseException], Optional[TracebackType], Optional[Thread]]):
+        @property
+        def exc_type(self) -> Type[BaseException]: ...
+        @property
+        def exc_value(self) -> Optional[BaseException]: ...
+        @property
+        def exc_traceback(self) -> Optional[TracebackType]: ...
+        @property
+        def thread(self) -> Optional[Thread]: ...
+    _excepthook: Callable[[_ExceptHookArgs], Any]