t.i.b.ThreadedResolver is now a new-style class

since 2009 / twisted-9.0.0
https://github.com/twisted/twisted/commit/663d669dce6ee5009eee8c6d2a81f5199855178b
Merge pull request #987 from nyov/nyov/resolver

t.i.b.ThreadedResolver is now a new-style class