mpi4py without mpi on system

A large number of warning make the CI difficult to read - most of them are of the form

/builds/core/dune-common/dune/common/simd/loop.hh:145: warning: ignoring '#pragma omp simd' [-Wunknown-pragmas]
/builds/core/dune-common/dune/common/simd/loop.hh:146: warning: ignoring '#pragma omp simd' [-Wunknown-pragmas]
  146 |     DUNE_SIMD_LOOP_ASSIGNMENT_OP(%=);

see e.g. https://gitlab.dune-project.org/core/dune-common/-/jobs/287920