Was anything beside _ and ... proposed?
Yes, the PEP mentions using '?'. It isn't the authors' first
choice but it seems they're not dead-set against it either.
Personally I prefer it to special-casing '_'. It has no meaning
in Python syntax yet, it would clearly not be an identifier (so no
one would be surprised that it doesn't bind), and it's inspired by
shell globbing. '*' already means "match more than one thing" in
several places, including PEP 622, and I'm guessing was also
inspired by shell globbing, so there's some mild precedence to
borrowing that syntax.