Make pixscale values robust to non-square pixels

[e-koch]

See astropy/astropy#13268.

We use pixscale = wcs.utils.proj_plane_pixel_scales(cube_hi.wcs.celestial)[0] throughout uvcombine and this poses the risk of not catching non-square pixels.

[keflavich]

it's such a minor thing here that I don't think we need to do anything about it right now, but I'm tired of using that really long and somewhat easy-to-break pattern