Update actions version to v4

.github/workflows/tests.yml

@@ -15,9 +15,9 @@ jobs:
     name: 'Build development docs'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Set up Python
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v4
         with:
           python-version: '3.x'
       - name: Install dependencies
@@ -37,7 +37,7 @@ jobs:
     name: 'Build user docs PDF'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Install fop
         run: |
           sudo apt-get install -y fop