argparse - make return type from `add_subparsers` public

This issue was brought up and closed as part of issue # 41592.

The add_subparsers method returns a protected _SubParsersAction instance. To add a sub-parser, the public add_parser method of the protected _SubParsersAction instance must be used.

1. It is unexpected to require the use of a public method of a protected class instance.
2. The protected return type breaks type hinting because
   1. protected and should not be imported
   2. it is not available for import

Real world use case example:

sub_parsers_action = parser.add_subparsers(...)

def build_subcmd(sub_parsers_action: _SubParsersAction):
    ...

The ask is to make the type of whatever is returned from add_subparsers public.

Linked PRs

• gh-157001

從 argparse.ArgumentParser.add_subparsers 開始，追蹤回傳的 _SubParsersAction，包括其公開的 add_parser 方法。檢視相關的 PR gh-157001 以及 argparse 周邊的 typing/API 情境；完成的條件是：無需匯入受保護的類別，即可將回傳的型別公開用於型別提示。