Here is a list of multiprocessing tests which are run in 3.8 but are not found in 3.9+:

OtherTest.test_answer_challenge_auth_failure
OtherTest.test_deliver_challenge_auth_failure
TestInitializers.test_manager_initializer
TestInitializers.test_pool_initializer
TestSyncManagerTypes.test_array
TestSyncManagerTypes.test_barrier
TestSyncManagerTypes.test_bounded_semaphore
TestSyncManagerTypes.test_condition
TestSyncManagerTypes.test_dict
TestSyncManagerTypes.test_event
TestSyncManagerTypes.test_joinable_queue
TestSyncManagerTypes.test_list
TestSyncManagerTypes.test_lock
TestSyncManagerTypes.test_namespace
TestSyncManagerTypes.test_pool
TestSyncManagerTypes.test_queue
TestSyncManagerTypes.test_rlock
TestSyncManagerTypes.test_semaphore
TestSyncManagerTypes.test_value
WithManagerTestBarrier.test_abort
WithManagerTestBarrier.test_abort_and_reset
WithManagerTestBarrier.test_action
WithManagerTestBarrier.test_barrier
WithManagerTestBarrier.test_barrier_10
WithManagerTestBarrier.test_default_timeout
WithManagerTestBarrier.test_reset
WithManagerTestBarrier.test_single_thread
WithManagerTestBarrier.test_thousand
WithManagerTestBarrier.test_timeout
WithManagerTestBarrier.test_wait_return
WithManagerTestCondition.test_notify
WithManagerTestCondition.test_notify_all
WithManagerTestCondition.test_notify_n
WithManagerTestCondition.test_timeout
WithManagerTestCondition.test_wait_result
WithManagerTestCondition.test_waitfor
WithManagerTestCondition.test_waitfor_timeout
WithManagerTestContainers.test_dict
WithManagerTestContainers.test_dict_iter
WithManagerTestContainers.test_dict_proxy_nested
WithManagerTestContainers.test_list
WithManagerTestContainers.test_list_iter
WithManagerTestContainers.test_list_proxy_in_list
WithManagerTestContainers.test_namespace
WithManagerTestEvent.test_event
WithManagerTestLock.test_lock
WithManagerTestLock.test_lock_context
WithManagerTestLock.test_rlock
WithManagerTestManagerRestart.test_rapid_restart
WithManagerTestMyManager.test_mymanager
WithManagerTestMyManager.test_mymanager_context
WithManagerTestMyManager.test_mymanager_context_prestarted
WithManagerTestPool.test_apply
WithManagerTestPool.test_async
WithManagerTestPool.test_async_timeout
WithManagerTestPool.test_context
WithManagerTestPool.test_empty_iterable
WithManagerTestPool.test_enter
WithManagerTestPool.test_imap
WithManagerTestPool.test_imap_handle_iterable_exception
WithManagerTestPool.test_imap_unordered
WithManagerTestPool.test_imap_unordered_handle_iterable_exception
WithManagerTestPool.test_make_pool
WithManagerTestPool.test_map
WithManagerTestPool.test_map_async
WithManagerTestPool.test_map_async_callbacks
WithManagerTestPool.test_map_chunksize
WithManagerTestPool.test_map_handle_iterable_exception
WithManagerTestPool.test_map_no_failfast
WithManagerTestPool.test_map_unplicklable
WithManagerTestPool.test_release_task_refs
WithManagerTestPool.test_resource_warning
WithManagerTestPool.test_starmap
WithManagerTestPool.test_starmap_async
WithManagerTestPool.test_terminate
WithManagerTestPool.test_traceback
WithManagerTestPool.test_wrapped_exception
WithManagerTestQueue.test_closed_queue_put_get_exceptions
WithManagerTestQueue.test_fork
WithManagerTestQueue.test_get
WithManagerTestQueue.test_no_import_lock_contention
WithManagerTestQueue.test_put
WithManagerTestQueue.test_qsize
WithManagerTestQueue.test_queue_feeder_donot_stop_onexc
WithManagerTestQueue.test_queue_feeder_on_queue_feeder_error
WithManagerTestQueue.test_task_done
WithManagerTestQueue.test_timeout
WithManagerTestRemoteManager.test_remote
WithManagerTestSemaphore.test_bounded_semaphore
WithManagerTestSemaphore.test_semaphore
WithManagerTestSemaphore.test_timeout
WithProcessesTestManagerRestart.test_rapid_restart
WithProcessesTestPicklingConnections.test_access
WithProcessesTestPicklingConnections.test_pickling
WithProcessesTestSharedMemory.test_shared_memory_ShareableList_basics
WithProcessesTestSharedMemory.test_shared_memory_ShareableList_pickling
WithProcessesTestSharedMemory.test_shared_memory_SharedMemoryManager_basics
WithProcessesTestSharedMemory.test_shared_memory_SharedMemoryManager_reuses_resource_tracker
WithProcessesTestSharedMemory.test_shared_memory_SharedMemoryServer_ignores_sigint
WithProcessesTestSharedMemory.test_shared_memory_across_processes
WithProcessesTestSharedMemory.test_shared_memory_basics
WithProcessesTestSharedMemory.test_shared_memory_cleaned_after_process_termination
WithThreadsTestManagerRestart.test_rapid_restart