Problems with parallel tests (MPI)

I'm experimenting problems with testing of parallel programs. In particular tests of the pysph package (https://salsa.debian.org/science-team/pysph) fail in CI with the following message:

There are not enough slots available in the system to satisfy the 2
slots that were requested by the application:
  /usr/bin/python3.9
Either request fewer slots for your application, or make more slots
available for use.
A "slot" is the Open MPI term for an allocatable unit where we can
launch a process.  The number of slots available are defined by the
environment in which Open MPI processes are run:
  1. Hostfile, via "slots=N" clauses (N defaults to number of
     processor cores if not provided)
  2. The --host command line parameter, via a ":N" suffix on the
     hostname (N defaults to 1 if not provided)
  3. Resource manager (e.g., SLURM, PBS/Torque, LSF, etc.)
  4. If none of a hostfile, the --host command line parameter, or an
     RM is present, Open MPI defaults to the number of processor cores
In all the above cases, if you want Open MPI to default to the number
of hardware threads instead of the number of processor cores, use the
--use-hwthread-cpus option.
Alternatively, you can use the --oversubscribe option to ignore the
number of available slots when deciding the number of processes to
launch.

The full build log is available at https://salsa.debian.org/science-team/pysph/-/jobs/1420077.

Please note that the package builds correctly in my local build environment and also after the upload the package builds without problems.

I suppose it is a limitation of the VM used for CI but I was not able to find a way to overcome this limitation.

Can you please provide some hint?