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The new API change looks doubtful for me. It is more verbose. And less readable. It makes harder to search in sources, because {'str', 'robuffer'} is the same as {'robuffer', 'str'}. If Argument Clinic would a tool with programmatic API, sets would make sense, but for now the main interface of Argument Clinic is text inclusions in C files.
If you want to convert the types parameter into a set, is it possible to continue to accept strings? Many user-friendly APIs accept either a sequence of string names or a string containing space-separated names. For example namedtuple, Enum.