Pin py-pynn to 0.9.5

spack.yaml

@@ -16,7 +16,7 @@ spack:
     - nest@3.0 +python +gsl +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 
+    - py-pynn@0.9.5 %gcc@10.3.0
     - tvb-data %gcc@10.3.0
     - tvb-library ^binutils+ld+gold %gcc@10.3.0
     - tvb-storage ^binutils+ld+gold %gcc@10.3.0