[issue41592] Make _SubParsersAction public

[paul j3]

paul j3 <ajipanca at gmail.com> added the comment:

`add_argument` also returns a Action subclass object.  All of those subclasses are "private".  While that return reference is usually ignored, sometimes it is useful to it, assigning it to a variable or list.  The documentation could be clearer about access to Action objects.

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nosy: +paul.j3

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41592>
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