[issue43160] argparse: add extend_const action

[paul j3]

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

It's not clear what your patch does that the existing 'store_const' doesn't.   Even 'append_const' does the same except for a extra level of list/tuple nesting.

But I'll admit that I didn't much need for  'extend' addition, but others, including the original developer liked it.

I suspect users of your addition will get a surprise if they aren't careful to provide a list or tuple 'const':

    const='foobar'
    const=str

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<https://bugs.python.org/issue43160>
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