TST: Test against pytest dev

[pllim]

As @bsipocz suggested, perhaps this packages needs to be tested against pytest dev (maybe as allowed failure). However, I am not sure how to implement it. Theoretically, it would be implemented over at ci-helpers but currently we have

https://github.com/astropy/ci-helpers/blob/661e07eb91dad30a3f0ccebd16e10c3c468c494e/travis/setup_dependencies_common.sh#L266-L270

Blocked by #95

[bsipocz]

good point about the pinning, I'll cook up something that works on the ci-helpers level, and not an utterly disgusting hack.

[pllim]

Looks like we do it already?