Add support for 32-bit linux

[astrofrog]

For several astropy packages I'd like to keep testing 32-bit linux. It might be a bit trickier to do this and we might need a special branch in the template, but I think it'd be worth it! We could always explicitly not support e.g. conda on there.

[Cadair]

@astrofrog this can be closed right?

[astrofrog]

Yep!