Update actions checkout and setup-python (fixes warning).

[fdmalone]

The mpi tests failing was a red herring. The examples were failing because of #278, this sent a signal to the other tests to fail but I'm guessing the error was only received on the root process so the MPI jobs would hang. I've set a timeout for these for the moment but there may be a more sensible thing to do.

[fdmalone]

I found several problems:

1. openmpi > 4.15 has an issue with bcast see: Deadlock in mpi_bcast using openmpi 4.1.5 from fortran open-mpi/ompi#11478. Using the fix suggested there in the mpirun launch fixes deadlocks.
2. example 02 is flaky (fails on CI (zero walker weight) but can't reproduce reliably locally.
3. mpich errors on finalize (so can't replace openmpi with mpich in CI).