Add checkout action

Author: JRMeyer

.github/workflows/build-and-test.yml

@@ -674,13 +674,13 @@ jobs:
     name: "Lin|Python notebook tests"
     runs-on: ubuntu-20.04
     if: ${{ github.event_name == 'pull_request' }}
-    strategy:
-      matrix:
-        pyver: [3.6, 3.7]
     steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 1
       - uses: actions/setup-python@v2
         with:
-          python-version: ${{ matrix.pyver }}
+          python-version: 3.8
       - name: Run python notebooks
         run: |
           sudo apt-get install -y --no-install-recommends libopusfile0 libopus-dev libopusfile-dev