Update Node.js 16 actions to Node.js 20 actions (#195)

This includes:
* actions/checkout@v3 to @v4.
* actions/setup-python@v4 to @v5.

Addresses the warning:
> Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: > actions/checkout@v3, actions/setup-python@v4. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

.github/workflows/ci.yml

@@ -11,9 +11,9 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Set up Python 3.8
-      uses: actions/setup-python@v4
+      uses: actions/setup-python@v5
       with:
         # pytype currently doesn't work with python >= 3.9
         python-version: 3.8
@@ -28,9 +28,9 @@ jobs:
         python-version: [3.8, 3.9, '3.10', '3.11']
         platform: [ubuntu-latest]
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v4
+      uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install tox
@@ -46,12 +46,12 @@ jobs:
       - test
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         # setuptools_scm requires the git clone to not be 'shallow'
         fetch-depth: 0
     - name: Set up Python
-      uses: actions/setup-python@v4
+      uses: actions/setup-python@v5
       with:
         python-version: "3.x"
     - name: Install dependencies