[issue43024] improve signature (in help, etc) for functions taking sentinel defaults

[Irit Katriel]

New submission from Irit Katriel <iritkatriel at yahoo.com>:

The "sentinel" default value don't render nicely in the signature:

>>> import traceback
>>> help(traceback.print_exception)
Help on function print_exception in module traceback:

print_exception(exc, /, value=<object object at 0x000002825DF09650>, tb=<object object at 0x000002825DF09650>, limit=None, file=None, chain=True)

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43024>
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