Make sure, python 3 is used in github action

macos runner used preinstalled 2.7 and did not find module pip.

.github/workflows/python-package.yml

@@ -30,7 +30,7 @@ jobs:
       uses: r-lib/actions/setup-r@v1
     - name: Install package with dependencies
       run: |
-        python -m pip install --upgrade pip
+        python3 -m pip install --upgrade pip
         pip install -e .[r_wrapper,tests,docs]
     - name: Lint with flake8
       run: |