Update actions/checkout action to v4

.github/workflows/format.yml

@@ -6,10 +6,10 @@ jobs:
   format:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           ref: ${{ github.head_ref }}
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Set up Python 3.7
         uses: actions/setup-python@v2
         with:

.github/workflows/pip-tests.yml

@@ -19,12 +19,12 @@ jobs:
           - os: macos-latest
             python-version: 3.8
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Set up Python 3.7
         uses: actions/setup-python@v2
         with:
           python-version: 3.7
-      - uses: actions/checkout@v2.4.0
+      - uses: actions/checkout@v4.1.5
       - name: Install Python dependencies
         run: |
           python -m pip install --upgrade pip

.github/workflows/tests.yml

@@ -14,12 +14,12 @@ jobs:
             python-version: 3.8
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Set up Python 3.7
       uses: actions/setup-python@v2
       with:
         python-version: 3.7
-    - uses: actions/checkout@v2.4.0
+    - uses: actions/checkout@v4.1.5
     - name: Install Python dependencies
       run: |
         python -m pip install --upgrade pip
@@ -47,12 +47,12 @@ jobs:
           - os: macos-latest
             python-version: 3.8
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Set up Python 3.7
         uses: actions/setup-python@v2
         with:
           python-version: 3.7
-      - uses: actions/checkout@v2.4.0
+      - uses: actions/checkout@v4.1.5
       - name: Install Python dependencies
         run: |
           python -m pip install --upgrade pip