Use action/cache v4 since v2 is end of life. (#155)

Author: nzlosh

.github/actions/py-dependencies/action.yaml

@@ -42,7 +42,7 @@ runs:
         python-version: '${{ inputs.python-version }}'
 
     - name: Cache Python Dependencies
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: |
           ~/.cache/pip