Fix pytester internal plugin to work correctly with latest versions of zope.interface

[nicoddemus]

There's a little hack on pytester that deletes modules from sys.modules that were imported during inline runs and it included a little nugget that prevented zope.interface from being deleted; apparently zope.interface keeps some internal state that can't just be deleted from sys.modules.

I guess at some point zope.interface was split into internal modules and the condition no longer had any effect. causing things to blow up spectacularly. 🔥

At first glance this seems like an internal testing issue and I wasn't going to add a CHANGELOG entry, but then realized that pytester is part our public API so it warrants an entry.

Fix #1989

[coveralls]

Coverage remained the same at 92.819% when pulling 2574da8 on nicoddemus:trial-envs into d15724f on pytest-dev:master.

[RonnyPfannschmidt]

@nicoddemus please rebase, also FML changelogs (i want the issue wrt changelogs gone)

[nicoddemus]

Oh my! Just used the new "Resolve Conflicts" button on the UI, very cool. Fixing CHANGELOG conflicts is now a breeze! 👍

[coveralls]

Coverage remained the same at 92.831% when pulling 3766060 on nicoddemus:trial-envs into e0c48b4 on pytest-dev:master.