argparse: add_mutually_exclusive_group

Vloot's entry point uses one (--console | --gui). The shim (lib/rtl/mimic_argparse.py) now has add_mutually_exclusive_group(required=) with CPython's rules: a member given a non-default value conflicts with another ("argument --gui: not allowed with argument --console"), a required group with no member set errors ("one of the arguments --json --text is required"), and usage shows [a | b] / (a | b). Messages, usage and help match python3 byte for byte (test_nilpy_argparse_mutually_exclusive_group, 9 command lines).