Upcoming incompatibility in pytest-xdist 2.0: "slave" aliases removed

[nicoddemus]

Hi!

As per pytest-dev/pytest-xdist#541, pytest-xdist 2.0 will remove the deprecated "slave"
aliases, which have been replaced by "worker" since pytest-xdist 1.22.

To avoid breakage, the team suggests to update your package to use the new terms, and make a new release to avoid sudden breakages once pytest-xdist 2.0 is released.

cc @Zac-HD @RonnyPfannschmidt

Thanks!

[asottile]

I can take this one