try to fix py2/3 ambiguity

spack.yaml

@@ -10,7 +10,7 @@ spack:
     # EBRAINS simulators
     #- nest@2.20.0 +python +gsl +mpi %gcc@10.3.0
     - nest@3.0 +python +gsl +mpi %gcc@10.3.0
-    - arbor +mpi %gcc@10.3.0
+    - arbor +mpi ^python@3:3.9 %gcc@10.3.0
     - neuron +mpi %gcc@10.3.0
     - py-pynn %gcc@10.3.0 
     # BrainScaleS (plus some constraining)