Update GitHub Actions versions, run flake8 on 3.12

This should fix

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

that you can see if you go look at the action runs on the web, which I
basically never do (e.g.
https://github.com/mgedmin/check-python-versions/actions/runs/8857533494)

And running flake8 with Python 3.12 allows it to check inside f-strings
due to AST parsing changes.

.github/workflows/build.yml

@@ -32,12 +32,12 @@ jobs:
         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: Pip cache
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: ~/.cache/pip
           key: ${{ runner.os }}-pip-${{ matrix.python-version }}-${{ hashFiles('setup.py') }}
@@ -85,13 +85,13 @@ jobs:
       - name: Git clone
         uses: actions/checkout@v4
 
-      - name: Set up Python ${{ env.default_python || '3.9' }}
-        uses: actions/setup-python@v4
+      - name: Set up Python ${{ env.default_python || '3.12' }}
+        uses: actions/setup-python@v5
         with:
-          python-version: "${{ env.default_python || '3.9' }}"
+          python-version: "${{ env.default_python || '3.12' }}"
 
       - name: Pip cache
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: ~/.cache/pip
           key: ${{ runner.os }}-pip-${{ matrix.toxenv }}-${{ hashFiles('tox.ini') }}