Remove pip cache fallback for CircleCI

This change removes the caching fallback in the case where dependencies
change, since it can cause CI failures when we have incompatible
dependencies in the cache.

.circleci/config.yml

           keys:
           # Parameterize the cache so that different python versions can get different versions of the packages
           - v1-dependencies-py<< parameters.pyversion >>-{{ checksum "python_deps.txt" }}
-          # fallback to using the latest cache if no exact match is found
-          - v1-dependencies-py<< parameters.pyversion >>-
 
       - run:
           name: Install Dependencies