github actions: use actions/checkout@v4

Fix warnings:
Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

.github/workflows/linux.yml

@@ -19,7 +19,7 @@ jobs:
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job
       # can access it
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - name: setup prerequisites (linux)
         shell: bash

.github/workflows/macos.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: macos-latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
     - name: Install build tools
       run: |

.github/workflows/windows.yml

@@ -13,7 +13,7 @@ jobs:
         windows-arch: ['x86', 'x64']
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v4