Fix circleCI meson error

Author: Ben Jeffery

.circleci/config.yml

@@ -7,7 +7,7 @@ jobs:
       - checkout
 
       - restore_cache:
-          key: tsinfer-{{ .Branch }}
+          key: tsinfer-{{ .Branch }}-v3
 
       - run:
           name: Install dependencies and set PATH
@@ -21,7 +21,9 @@ jobs:
       - run:
           name: Install development dependencies
           command: |
+              pyenv global 3.8
               pip install -r requirements/CI-tests-complete/requirements.txt --user
+              pyenv rehash
 
       - save_cache:
           key: tsinfer-{{ .Branch }}