Revert "Remove cacheing"

This reverts commit 653322e.

Author: SimmonsRitchie

.github/workflows/ci.yml

@@ -28,6 +28,15 @@ jobs:
       - name: Install Pipenv
         uses: dschep/install-pipenv-action@v1
 
+      - name: Cache Python dependencies
+        uses: actions/cache@v1
+        with:
+          path: .venv
+          key: pip-${{ matrix.python-version }}-${{ hashFiles('**/Pipfile.lock') }}
+          restore-keys: |
+            pip-${{ matrix.python-version }}-
+            pip-
+
       - name: Install dependencies
         run: pipenv sync --dev
         env: