Does not work with Python 3.14, breaking changes to Argparse

[mmrwoods]

All CLI commands are broken when using Python 3.14 due to changes in Argparse, adding nested mutually exclusive groups is now an error:

$ aws-encryption-cli --version --verbosity 4
Encountered unexpected error: increase verbosity to see details.
ValueError("mutually exclusive groups cannot be nested")

Exception traceback:

  File "/Users/xxxxxx/.local/bin/aws-encryption-cli", line 7, in <module>
    sys.exit(cli())
             ~~~^^
  File "/Users/xxxxxx/.local/pipx/venvs/aws-encryption-sdk-cli/lib/python3.14/site-packages/aws_encryption_sdk_cli/__init__.py", line 270, in cli
    args = parse_args(raw_args)
  File "/Users/xxxxxx/.local/pipx/venvs/aws-encryption-sdk-cli/lib/python3.14/site-packages/aws_encryption_sdk_cli/internal/arg_parsing.py", line 636, in parse_args
    parser = _build_parser()
  File "/Users/xxxxxx/.local/pipx/venvs/aws-encryption-sdk-cli/lib/python3.14/site-packages/aws_encryption_sdk_cli/internal/arg_parsing.py", line 186, in _build_parser
    operating_action = version_or_action.add_mutually_exclusive_group()
  File "/opt/homebrew/Cellar/python@3.14/3.14.0_1/Frameworks/Python.framework/Versions/3.14/lib/python3.14/argparse.py", line 1816, in add_mutually_exclusive_group
    raise ValueError('mutually exclusive groups cannot be nested')
ValueError: mutually exclusive groups cannot be nested

In Python 3.14 the relevant code in argparse.py is:

class _MutuallyExclusiveGroup(_ArgumentGroup):
    ...
    def add_mutually_exclusive_group(self, **kwargs):
        raise ValueError('mutually exclusive groups cannot be nested')

At a glance, I'm not sure how to fix this, it looks to require a bit of an overhaul of argument parsing. Nesting mutually exclusive groups has been deprecated since Python 3.11 though.