Install python manually in the unit test container

.circleci/config.yml

@@ -2,6 +2,7 @@ version: 2
 jobs:
     unit-test:
         docker:
+            # Match target OS of TLJH
             - image: ubuntu:18.04
 
         working_directory: ~/repo
@@ -9,6 +10,12 @@ jobs:
         steps:
             - checkout
 
+            # Setup Python
+            - run:
+                name: install python
+                command: |
+                    apt-get update --yes && apt-get install --yes python3 python3-pip
+
             # Download and cache dependencies
             - restore_cache:
                 keys: