pin pyrsistent version to 0.16.0 for python2 compatibility

Author: ranman

.circleci/config.yml

@@ -25,7 +25,7 @@ jobs:
           command: yarn install --cache-folder ~/.cache/yarn
       - run:
           name: Notedown Install
-          command: sudo apt update && sudo apt install python-pip && sudo -H pip install notedown pyyaml -Iv nbformat==4.4.0
+          command: sudo apt update && sudo apt install python-pip && sudo -H pip install pyrsistent==0.16 notedown pyyaml -Iv nbformat==4.4.0
       - save_cache:
           key: pytorch-yarn-{{ checksum "yarn.lock" }}
           paths: