Sigmoid Function

Implement:

def sigmoid(z: np.ndarray) -> np.ndarray:
    """
    z: any shape of real numbers.
    Returns elementwise sigmoid, same shape as z, values strictly in (0, 1).
    """

Your function should:

1. Work elementwise on arrays of any shape.
2. Never overflow or return nan, even for very large |z|.
3. Return exactly 0.5 when z = 0.