IMO it's ok to cast C signint() int result into a Python boolean, since it's unlikely that the signbit() will return values others than 0 and 1 in the future. The manual page says:

Actually,signbit does return something else than 0/1 (in C) at least on some platforms, but that's what all predicate functions do in math.h (they are all specified as returning nonzero value if the condition holds). However, in C++, they return true/false. Note thathttps://en.cppreference.com/w/c/numeric/math/signbit shows an example where signbit() returns 128 for a negative zero.

However, since we return booleans for isfinite() etc, I also returned a boolean for signbit() (and in practice, people don't care about the return value, they only care about the test)