Add __all__ to modules beginning with 'm' (#7330)

stdlib/multiprocessing/spawn.pyi

@@ -1,6 +1,16 @@
 from types import ModuleType
 from typing import Any, Mapping, Sequence
 
+__all__ = [
+    "_main",
+    "freeze_support",
+    "set_executable",
+    "get_executable",
+    "get_preparation_data",
+    "get_command_line",
+    "import_main_path",
+]
+
 WINEXE: bool
 WINSERVICE: bool