Fix pep508_rs doc test

[konstin]

Since nextest does not run doctests, this did not show up on CI.

[BurntSushi]

Hmmm I don't think this --features all-tests does the same thing as --all-features. I think you'd need to add an all-tests feature to every crate duplicating all of the feature names in that crate (sans any debug features).

[konstin]

I've trimmed this down to only fix the pep508_rs doc test