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Thing just getting weird. If we remove the guard, some of the builtin function will get not-so-good signature result:
>>> i.signature(range.__init__)
<Signature (self, /, *args, **kwargs)>
>>> i.signature(list.append) 
<Signature (self, object, /)>
>>> i.signature([].append) 
<Signature (object, /)>
You can check my latest commit, The most big change is about __method__, it will make some different behavior:
str(i.signature(list.__new__)) -> (*args, **kwargs)
str(i.signature(p.sub)) -> (repl, string, count=0)
both of them are BuiltinFunctionType, and the first one in old version will be guard, so the get_argspec result will be docstring, we can't different them in non-guard version.