Update spack.yaml

spack.yaml

-
+spack:
+  specs:
+    - nest +python +gsl +mpi ^py-scipy@:1.6.999 %gcc@10.3.0
+    - arbor +mpi %gcc@10.3.0
+    - neuron +mpi %gcc@10.3.0
+    # BrainScaleS (plus some constraining)
+    - visionary-dls-core ^py-setuptools-scm+toml ^python@3.8.0:3.8.999 ^binutils+ld ^py-astroid@2.5.6:2.6.999 ^py-scipy@:1.6.999 %gcc@10.3.0