Deprecate prefix_chars in argument_group

[savannahostrowski]

Now, after conflict_handler and argument_default have been documented, we can deprecate prefix_chars. @savannahostrowski, are you interesting? We should just add a runtime warning in the _ArgumentGroup constructor, test, and documentation (the deprecated instruction and something in What's New).

Originally posted by @serhiy-storchaka in #89819 (comment)

https://docs.python.org/3/library/argparse.html#argument-groups

Linked PRs

• gh-125542: Deprecate prefix_chars in ArgumentParser.add_argument_group #125563

[serhiy-storchaka]

Strictly, speaking, there was no need to open a new issue, as this change is the solution for #89819.

[savannahostrowski]

D'oh! My bad—I must have mistakenly done this, thinking the other issue was closed after I documented the other two parameters!