bpo-43220: Accept explicit default args in required groups

[keith]

Previously if you had a default argument on a required mutually
exclusive group and the default was explicitly passed, it was treated
as if no value was passed. Now it accepts that value.

https://bugs.python.org/issue43220

[the-knights-who-say-ni]

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[rhettinger]

Please add a NEWS entry using Blurb.

[keith]

Updated!

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[keith]

Any idea who I can get to review?

[keith]

@rhettinger friendly bump!

[furkanonder]

LGTM.

[keith]

I merged main into this to resolve the conflict. @furkanonder should I do anything else here or is the process to wait for further review?

[serhiy-storchaka]

Thank you for your contribution. It was fixed in other way by #124307.