Update actions/checkout action to v4

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
   runs-on: ubuntu-latest
   steps:
   - name: Checkout
-    uses: actions/checkout@v2
+    uses: actions/checkout@v4
   - name: Set up Python 3.9
     uses: actions/setup-python@v2
     with:
@@ -23,7 +23,7 @@ jobs:
   runs-on: ${{ matrix.os }}
   steps:
   - name: Checkout
-    uses: actions/checkout@v2
+    uses: actions/checkout@v4
   - name: Set up ffmpeg
     uses: FedericoCarboni/setup-ffmpeg@v2
   - name: Set up Python 3.9