Improve multiprocessing stubs

首先运行 issue 的 python3.8 -m mypy.stubtest --custom-typeshed-dir . multiprocessing --concise 命令，并检查与一小组已报告不匹配项对应的 multiprocessing 存根。将选定的 API 与运行时进行比较，仅更新相关存根，然后重新运行 stubtest，以确认这些差异已解决。

stubtest finds a number of issues with the existing multiprocessing stubs. PRs that work toward fixing these (if they need fixing) are welcome, even if they're small!

~/dev/typeshed master λ python3.8 -m mypy.stubtest --custom-typeshed-dir . multiprocessing --concise                     
multiprocessing.Array is not a type
multiprocessing.Event is inconsistent, runtime does not have argument "lock"
multiprocessing.JoinableQueue is not a type
multiprocessing.Queue is not a type
multiprocessing.RawArray is not present in stub
multiprocessing.RawValue is not present in stub
multiprocessing.SimpleQueue is not a type
multiprocessing.Value is not a type
multiprocessing.reducer is not present in stub
multiprocessing.set_executable is inconsistent, stub argument "exe" differs from runtime argument "executable"
multiprocessing.context.BaseContext.Event is inconsistent, runtime does not have argument "lock"
multiprocessing.dummy.Namespace.__getattr__ is not present at runtime
multiprocessing.dummy.Pipe is not present in stub
multiprocessing.managers.BaseManager.shutdown is not present at runtime
multiprocessing.managers.Namespace.__getattr__ is not present at runtime
multiprocessing.managers.Server.__init__ is inconsistent, stub does not have argument "registry"
multiprocessing.managers.Server.__init__ is inconsistent, stub does not have argument "address"
multiprocessing.managers.Server.__init__ is inconsistent, stub does not have argument "authkey"
multiprocessing.managers.Server.__init__ is inconsistent, stub does not have argument "serializer"
multiprocessing.managers.Server.accept_connection is not present in stub
multiprocessing.managers.Server.accepter is not present in stub
multiprocessing.managers.Server.create is not present in stub
multiprocessing.managers.Server.debug_info is not present in stub
multiprocessing.managers.Server.decref is not present in stub
multiprocessing.managers.Server.dummy is not present in stub
multiprocessing.managers.Server.fallback_getvalue is not present in stub
multiprocessing.managers.Server.fallback_mapping is not present in stub
multiprocessing.managers.Server.fallback_repr is not present in stub
multiprocessing.managers.Server.fallback_str is not present in stub
multiprocessing.managers.Server.get_methods is not present in stub
multiprocessing.managers.Server.handle_request is not present in stub
multiprocessing.managers.Server.incref is not present in stub
multiprocessing.managers.Server.number_of_objects is not present in stub
multiprocessing.managers.Server.public is not present in stub
multiprocessing.managers.Server.serve_client is not present in stub
multiprocessing.managers.Server.shutdown is not present in stub
multiprocessing.managers.SharedMemoryServer.__init__ is inconsistent, stub does not have *args argument "args"
multiprocessing.managers.SharedMemoryServer.create is not present in stub
multiprocessing.managers.SharedMemoryServer.list_segments is not present in stub
multiprocessing.managers.SharedMemoryServer.public is not present in stub
multiprocessing.managers.SharedMemoryServer.release_segment is not present in stub
multiprocessing.managers.SharedMemoryServer.shutdown is not present in stub
multiprocessing.managers.SharedMemoryServer.track_segment is not present in stub
multiprocessing.managers.SyncManager.Barrier is not present in stub
multiprocessing.managers.SyncManager.Event is inconsistent, stub does not have *args argument "args"
multiprocessing.managers.SyncManager.JoinableQueue is not present in stub
multiprocessing.managers.SyncManager.Lock is inconsistent, stub does not have *args argument "args"
multiprocessing.managers.SyncManager.Namespace is inconsistent, stub does not have *args argument "args"
multiprocessing.managers.SyncManager.Pool is not present in stub
multiprocessing.managers.SyncManager.RLock is inconsistent, stub does not have *args argument "args"
multiprocessing.pool.ApplyResult.__init__ is inconsistent, stub does not have argument "pool"
multiprocessing.pool.ApplyResult.__init__ is inconsistent, stub does not have argument "callback"
multiprocessing.pool.ApplyResult.__init__ is inconsistent, stub does not have argument "error_callback"
multiprocessing.pool.CLOSE variable differs from runtime type Literal['CLOSE']
multiprocessing.pool.IMapIterator.__init__ is inconsistent, stub does not have argument "pool"
multiprocessing.pool.MapResult.__init__ is inconsistent, stub does not have argument "pool"
multiprocessing.pool.MapResult.__init__ is inconsistent, stub does not have argument "chunksize"
multiprocessing.pool.MapResult.__init__ is inconsistent, stub does not have argument "length"
multiprocessing.pool.MapResult.__init__ is inconsistent, stub does not have argument "callback"
multiprocessing.pool.MapResult.__init__ is inconsistent, stub does not have argument "error_callback"
multiprocessing.pool.Pool.Process is not present in stub
multiprocessing.pool.Pool.imap is inconsistent, stub argument "iterable" has a default value but runtime argument does not
multiprocessing.pool.Pool.imap_unordered is inconsistent, stub argument "iterable" has a default value but runtime argument does not
multiprocessing.pool.Pool.map is inconsistent, stub argument "iterable" has a default value but runtime argument does not
multiprocessing.pool.Pool.map_async is inconsistent, stub argument "iterable" has a default value but runtime argument does not
multiprocessing.pool.Pool.starmap is inconsistent, stub argument "iterable" has a default value but runtime argument does not
multiprocessing.pool.Pool.starmap_async is inconsistent, stub argument "iterable" has a default value but runtime argument does not
multiprocessing.pool.RUN variable differs from runtime type Literal['RUN']
multiprocessing.pool.TERMINATE variable differs from runtime type Literal['TERMINATE']
multiprocessing.pool.ThreadPool.Process is not present in stub
multiprocessing.queues.JoinableQueue.__init__ is inconsistent, stub argument "ctx" has a default value but runtime argument does not
multiprocessing.queues.Queue.__init__ is inconsistent, stub argument "ctx" has a default value but runtime argument does not
multiprocessing.queues.Queue.put_nowait is inconsistent, stub argument "item" differs from runtime argument "obj"
multiprocessing.queues.SimpleQueue.__init__ is inconsistent, stub argument "ctx" has a default value but runtime argument does not
multiprocessing.queues.SimpleQueue.put is inconsistent, stub argument "item" differs from runtime argument "obj"
multiprocessing.spawn._main is inconsistent, stub does not have argument "parent_sentinel"
multiprocessing.synchronize.Barrier.__init__ is inconsistent, runtime does not have *args argument "ctx"
multiprocessing.synchronize.Barrier.__init__ is inconsistent, stub does not have argument "ctx"
multiprocessing.synchronize.Condition.acquire is not present at runtime
multiprocessing.synchronize.Condition.release is not present at runtime
multiprocessing.synchronize.Event.__init__ is inconsistent, runtime does not have argument "lock"
multiprocessing.synchronize.SemLock.__init__ is inconsistent, stub does not have argument "kind"
multiprocessing.synchronize.SemLock.__init__ is inconsistent, stub does not have argument "value"
multiprocessing.synchronize.SemLock.__init__ is inconsistent, stub does not have argument "maxvalue"
multiprocessing.synchronize.SemLock.__init__ is inconsistent, stub does not have argument "ctx"
multiprocessing.synchronize.SemLock.acquire is not present at runtime
multiprocessing.synchronize.SemLock.release is not present at runtime
multiprocessing.synchronize.Semaphore.get_value is not present in stub