Function py_kopf_kmatrix_pi1 

pub fn py_kopf_kmatrix_pi1(
    tags: &Bound<'_, PyTuple>,
    couplings: [[PyParameter; 2]; 1],
    channel: PyKopfKMatrixPi1Channel,
    mass: PyMass,
) -> PyResult<PyExpression>

Construct the fixed Kopf pi1 K-matrix amplitude.