Remove ubuntu-20.04, python 3.6 and python 3.7 from CI

Ubuntu 20.04 only has libxkbcommon-0.8, and we are preparing to require
libxkbcommon-1.0

Python 3.6 and Python 3.7 are no longer supported.

Add Python 3.12 to CI

Update action scripts to avoid deprecation warnings

.github/workflows/run-tests.yml

@@ -7,21 +7,18 @@ jobs:
 
     strategy:
       matrix:
-        os: ['ubuntu-20.04', 'ubuntu-22.04']
-        python-version: ['3.6', '3.7', '3.8', '3.9', '3.10', '3.11']
-        exclude:
-          - os: 'ubuntu-22.04'
-            python-version: '3.6'
+        os: ['ubuntu-22.04']
+        python-version: ['3.8', '3.9', '3.10', '3.11', '3.12']
 
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install libxkbcommon-dev
         run: |
           sudo apt-get install libxkbcommon-dev
       - 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 dependencies