New try at including Python 3.5 in GitHub Actions CI

.github/workflows/python-package.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.5, 3.6, 3.7, 3.8]
 
     steps:
     - uses: actions/checkout@v2
@@ -26,7 +26,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install flake8 astropy pytest
+        pip install flake8 astropy pytest scons
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
 #    - name: Lint with flake8
 #      run: |