Fix CP #34742: {}.update(b=1) -> TypeError

b was used as the name of the argument to the C# update() function,
which confused named argument resolution. Changed to a less likely name
(otherø aka other\u00F8) following a pattern used in other, similar
functions.

(cherry picked from commit 356b9e10572b2ec119d2a854e2f5ccce8ea56b02)