Fix GitHub Actions workflow to support Python 3.6-3.14

Use Docker containers with official Python images to run tests across
all Python versions from 3.6 to 3.14. This approach:

- Eliminates "workers not found" errors from deprecated Ubuntu runners
- Provides reliable support for older Python versions (3.6, 3.7)
- Simplifies workflow configuration significantly
- Uses official python:X.Y Docker images maintained by Python team
- Sets fail-fast: false to test all versions independently

.github/workflows/run_tests.yml

@@ -3,21 +3,23 @@ name: Run tests
 on: [push]
 
 jobs:
-  build:
-    runs-on: ubuntu-20.04
+  test:
+    runs-on: ubuntu-latest
+    container:
+      image: python:${{ matrix.python-version }}
     strategy:
+      fail-fast: false
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11"]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
+
     steps:
     - uses: actions/checkout@v4
-    - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v4
-      with:
-        python-version: ${{ matrix.python-version }}
+
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
         pip install -r requirements-dev.txt
+
     - name: Run tests
       run: |
         python -m pytest -v