Use python3.9 instead of 3.11 since the latter is not available on all surf machines

aggregate_and_plot.sh

@@ -4,7 +4,7 @@ if [ -d "toil_venv" ]; then
     source toil_venv/bin/activate
 else
     echo "Creating new virtual environment for using toil"
-    python3.11 -m venv toil_venv
+    python3.9 -m venv toil_venv
     source toil_venv/bin/activate
     pip install --upgrade pip setuptools wheel
     pip install toil[cwl] singularity