MAINT: stats.variation: add/use temporary xp_copysign

[mdhaber]

Reference issue

scipy#20647

What does this implement/fix?

Adds an xp_copysign to pass tests with array_api_strict.

Additional information

I don't expect it to be a big problem for this to ignore the special cases, but if you prefer, you can add them or just skip tests with array_api_strict until array_api_compat adds this.

I also took the opportunity to change the name of _move_axis_to_end to something that is more recognizable as a temporary array API function.

[j-bowhay]

Thanks!