dummy_thread/ing: remove in 3.9 (#6408)

https://bugs.python.org/issue37312

stdlib/VERSIONS

@@ -29,8 +29,8 @@ _compression: 3.6-
 _csv: 2.7-
 _curses: 2.7-
 _decimal: 3.6-
-_dummy_thread: 3.6-
-_dummy_threading: 2.7-
+_dummy_thread: 3.6-3.8
+_dummy_threading: 2.7-3.8
 _heapq: 2.7-
 _imp: 3.6-
 _json: 2.7-
@@ -105,7 +105,7 @@ dis: 2.7-
 distutils: 2.7-
 distutils.command.bdist_msi: 2.7-3.10
 doctest: 2.7-
-dummy_threading: 2.7-
+dummy_threading: 2.7-3.8
 email: 2.7-
 encodings: 2.7-
 ensurepip: 2.7-