Disallow pytest-xdist parallel testing

[kmpaul]

Mpirical doesn't seem to play nicely with pytest-xdist. It does not seem to be the case that simply installing pytest-xdist is a problem, but using certain features, such as the -n N option appears to not play nicely with Mpirical's MPI-based parallelism. Is there a way to disallow the -n usage when mpirical is installed? Would this require a pytest-mpirical plugin?