Bug#978066: pmix: breaks openmpi autopkgtest

[Graham Inggs]

Control: reopen -1

pmix/4.0.0-2 from unstable still causes the autopkgtests of openmpi
4.0.5-7 in testing to fail

If it is not intended that this combination should work together,
please add Breaks: libopenmpi3 (<< 4.1.0-3) or similar to libpmix2.