`fields.effective_cls` is still calling np

[connoraird]

Describe the Bug

Although fields.effective_cls was ported in #407 np is still being called directly...

pairs = (
    combinations_with_replacement(np.ndindex(shape1[1:]), 2)
    if weights2 is weights1
    else product(np.ndindex(shape1[1:]), np.ndindex(shape2[1:]))
)

To Reproduce

Expected Behaviour

We should not be calling np but instead one of xp, uxpx or xpx

Actual Behaviour

No response

Version In Use

main

Additional Context

[paddyroddy]

Seems this is blocked by ndindex not being in the Array API