Upgrade actions/checkout to v4. (#391)

Author: benjaminp

.github/workflows/ci.yml

@@ -61,7 +61,7 @@ jobs:
     env:
       TOXENV: py
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - if: ${{ matrix.python-version == '2.7' }}
         run: |
           sudo apt-get install python-is-python2

.github/workflows/publish.yml

@@ -10,7 +10,7 @@ jobs:
   deploy:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Set up Python
       uses: actions/setup-python@v5
       with: