Cache dependencies, although the gain is insignificant for this repo

Author: rayluo

.github/workflows/python-package.yml

@@ -30,6 +30,20 @@ jobs:
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
+
+    # Derived from https://github.com/actions/cache/blob/main/examples.md#using-pip-to-get-cache-location
+    # However, a before-and-after test shows no improvement in this repo,
+    # possibly because the bottlenect was not in downloading those small python deps.
+    - name: Get pip cache dir from pip 20.1+
+      id: pip-cache
+      run: |
+        echo "::set-output name=dir::$(pip cache dir)"
+    - name: pip cache
+      uses: actions/cache@v2
+      with:
+        path: ${{ steps.pip-cache.outputs.dir }}
+        key: ${{ runner.os }}-py${{ matrix.python-version }}-pip-${{ hashFiles('**/setup.py', '**/requirements.txt') }}
+
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip