Tests pass on pypy

This is https://github.com/Supervisor/supervisor/pull/425 applied to `master`.

Make test_socket_manager tests pass on PyPy

by applying patch from @pjenvey on IRC:
http://paste.ofcode.org/a588X46F8gNdFDSHDtK5NP

Cc: @mcdonc, @gcarothers, @mnaberez , @pjenvey