Make _MutuallyExclusiveGroup a public API in argparse

[gaborbernat]

argparse.ArgumentParser.add_mutually_exclusive_group() returns an argparse._MutuallyExclusiveGroup instance, but this type is private (underscore-prefixed). This makes it impossible to properly type-annotate code that stores the return value without importing a private symbol.

Motivation

When using type checkers (mypy, ty, pyright), code like:

group = parser.add_argument_group("selection")
add_to: ArgumentParser = group.add_mutually_exclusive_group(required=False)

is flagged as a type error because _MutuallyExclusiveGroup is not ArgumentParser, but there's no public type to use instead. The only workaround is importing the private _MutuallyExclusiveGroup.

This is similar to #101503 which requests making the return type of add_subparsers public.

Proposal

Make _MutuallyExclusiveGroup (and possibly _ArgumentGroup) public by removing the underscore prefix, or at minimum export a public type alias.