a.reshape(3, 3, -1).dot(rotmat).reshape(-1, 9)
array([[0.0, 1.0, 0.0, -1.0, 1.0, 1.0, -1.0, 1.0, 0.0], [-1.0, 0.0, 1.0, 
    -1.0, 1.0, 1.0, -1.0, 0.0, 0.0], [0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 
    1.0, 1.0]])
Most efficient row multiplication with matrix in Pandas