add cache pip

.github/workflows/python_package.yml

@@ -19,6 +19,14 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
+      - name: Cache pip
+        uses: actions/cache@v2
+        with:
+          path: ~/.cache/pip
+          key: ${{ runner.os }}-pip-${{ hashFiles('**/pyproject.toml') }}
+          restore-keys: |
+            ${{ runner.os }}-pip-
+
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip