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.

tests/stubtest_whitelists/py38.txt

@@ -1,5 +1,3 @@
-_thread.ExceptHookArgs
-_thread._ExceptHookArgs
 ast.Bytes.__new__
 ast.Ellipsis.__new__
 ast.NameConstant.__new__

tests/stubtest_whitelists/py39.txt

@@ -1,7 +1,5 @@
 _ast.ImportFrom.level
 _dummy_thread
-_thread.ExceptHookArgs
-_thread._ExceptHookArgs
 ast.Bytes.__new__
 ast.Ellipsis.__new__
 ast.ExtSlice.__new__