xsdk: update to mfem and py-petsc4py changes (#16378)
xsdk: update to mfem and py-petsc4py changes (#16378)

mfem variant hypre is now rolled into variant mpi - so update spec accordingly

mfem@4.0.1-xsdk+superlu-dist is broken and unsupported - so disable it

with the addition of py-petsc4py@3.13.0 - conretizer gets confused and is not picking py-petsc4py@3.12.0 as a compatible dependency with petsc@3.12. So manually specify it.
Also depends_on('py-libensemble@0.5.2+petsc4py ^py-petsc4py@3.12.0' causes concretizer to hang forever
iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack