bpo-39481: PEP 585 for dataclasses, mailbox, contextvars (GH-19425)

diff --git a/Lib/test/test_genericalias.py b/Lib/test/test_genericalias.py
index 686df17..37cbf92 100644
--- a/Lib/test/test_genericalias.py
+++ b/Lib/test/test_genericalias.py
@@ -9,7 +9,10 @@
 from concurrent.futures import Future
 from concurrent.futures.thread import _WorkItem
 from contextlib import AbstractContextManager, AbstractAsyncContextManager
-from functools import partial, partialmethod, _lru_cache_wrapper, cached_property
+from contextvars import ContextVar, Token
+from dataclasses import Field
+from functools import partial, partialmethod, cached_property
+from mailbox import Mailbox, _PartialFile
 from ctypes import Array, LibraryLoader
 from difflib import SequenceMatcher
 from filecmp import dircmp
@@ -60,6 +63,9 @@
                   Reversible,
                   Container, Collection,
                   Callable,
+                  Mailbox, _PartialFile,
+                  ContextVar, Token,
+                  Field,
                   Set, MutableSet,
                   Mapping, MutableMapping, MappingView,
                   KeysView, ItemsView, ValuesView,