[RClaudiuM]

Fixes #368

On Python 3.14, every simple_parsing.ArgumentParser emits:

DeprecationWarning: The use of the undocumented 'prefix_chars' parameter in ArgumentParser.add_argument_group() is deprecated.

ArgumentParser.add_argument_group always forwarded prefix_chars=prefix_chars or self.prefix_chars, and argparse 3.14 warns whenever the keyword is present. When prefix_chars is omitted, argparse's _ArgumentGroup already falls back to the parser's prefix_chars (this is true on every supported Python version), so the override now forwards it only when the caller passes one explicitly. argument_default and conflict_handler are handled as before.

Added test/test_issue_368.py:

• parsing a dataclass with DeprecationWarning turned into an error must not raise;
• a group still inherits a custom prefix_chars / argument_default / conflict_handler from the parser.

Testing

• pytest test/test_issue_368.py on 3.14: fails before the change (DeprecationWarning), passes after. Passes on 3.12 both before and after.
• Full suite on 3.12 (uv sync --all-extras): same results before and after. The one failure, test_bools.py::test_bool_args_in_help, already fails on master.
• Full suite on 3.14: the same 22 tests fail before and after (help text changes in 3.14 argparse, not related to this). With -W error::DeprecationWarning, it goes from 1006 failures to those same 22.
• pre-commit run --all-files passes.