As a test, gcc and clang both seem happy to treat this as valid C.  I think Sphinx is wrong to reject this.

mdickinson@mirzakhani Desktop % cat test.c
typedef struct {
    double real;
    double imag;
} Py_complex;

Py_complex _Py_c_neg(Py_complex complex);
mdickinson@mirzakhani Desktop % gcc -Wall -Wextra -std=c17 -c test.c
mdickinson@mirzakhani Desktop % clang -Wall -Wextra -std=c17 -c test.c
mdickinson@mirzakhani Desktop %