fix python version variable

Author: ericmjl

.azure-pipelines/templates/setup-script.yml

@@ -2,6 +2,6 @@ steps:
 - script: |
     conda env create -f environment.yml
     source activate bayesian-modelling-tutorial
-    conda install -y python=${{ python.version }}
+    conda install -y python=$(python.version)
     python -m ipykernel install --user --name bayesian-modelling-tutorial
   displayName: 'Create environment, install correct Python, and activate kernel.'