enh: cache pip downloaded packages

Author: oesteban

.github/workflows/pythonpackage.yml

@@ -31,6 +31,12 @@ jobs:
       uses: actions/setup-python@v1
       with:
         python-version: ${{ matrix.python-version }}
+    - uses: actions/cache@v1
+      with:
+        path: $HOME/.cache/pip
+        key: pip-cache-v1
+        restore-keys: |
+          pip-cache-
 
     - name: Build in confined environment and interpolate version
       run: |