Pin conda OpenMPI to non-external

.test-conda-env-py3.yml

@@ -17,3 +17,11 @@ dependencies:
 
 # Only needed to make pylint succeed
 - matplotlib-base
+
+# This is intended to prevent conda from selecting 'external' (i.e. empty) builds
+# of OpenMPI to satisfy the MPI dependency of mpi4py. It did so in May 2024, leading
+# to confusing failues saying
+# 'libmpi.so.40: cannot open shared object file: No such file or directory'.
+# https://github.com/conda-forge/openmpi-feedstock/issues/153
+# https://conda-forge.org/docs/user/tipsandtricks/#using-external-message-passing-interface-mpi-libraries
+- openmpi>=5=h*