ci: update GitHub Actions to Node.js 20+ compatible versions

Bump actions/checkout@v3 -> v4 and actions/setup-python@v3 -> v5
in both flake8.yml and tests.yml workflows to resolve the
Node.js 20 deprecation warning on GitHub Actions runners.

Author: justinbacle

.github/workflows/flake8.yml

@@ -9,9 +9,9 @@ jobs:
       matrix:
         python-version: ["3.13"]
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v3
+      uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies

.github/workflows/tests.yml

@@ -9,9 +9,9 @@ jobs:
       matrix:
         python-version: ["3.13"]
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v3
+      uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies