Add _threading_local.local.__delattr__ back to the stub (#8526)

test_cases/stdlib/test_threading.py

@@ -0,0 +1,12 @@
+import _threading_local
+import threading
+
+loc = threading.local()
+loc.foo = 42
+del loc.foo
+loc.baz = ["spam", "eggs"]
+del loc.baz
+
+l2 = _threading_local.local()
+l2.asdfasdf = 56
+del l2.asdfasdf