gh-111388: Add show_group to traceback.format_exception_only

[sobolevn]

This is the first step to #104150

• Issue: Add show_group param to traceback.format_exception_only function #111388

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📚 Documentation preview 📚: https://cpython-previews--111390.org.readthedocs.build/

[iritkatriel]

Does this fix need to be backported to 3.12/311?

[sobolevn]

No, because 3.12 does not have show_group / _depth at all:

cpython/Lib/traceback.py

Lines 865 to 892 in 307ca78

	def format_exception_only(self):
	"""Format the exception part of the traceback.
	The return value is a generator of strings, each ending in a newline.
	Generator yields the exception message.
	For :exc:`SyntaxError` exceptions, it
	also yields (before the exception message)
	several lines that (when printed)
	display detailed information about where the syntax error occurred.
	Following the message, generator also yields
	all the exception's ``__notes__``.
	"""
	if self.exc_type is None:
	yield _format_final_exc_line(None, self._str)
	return
	stype = self.exc_type.__qualname__
	smod = self.exc_type.__module__
	if smod not in ("__main__", "builtins"):
	if not isinstance(smod, str):
	smod = "<unknown>"
	stype = smod + '.' + stype
	if not issubclass(self.exc_type, SyntaxError):
	yield _format_final_exc_line(stype, self._str)
	else:
	yield from self._format_syntax_error(stype)

And without it, the result is identical with and without this fix.