Run test matrix in python version containers

ubuntu-latest (24.04) no longer provides a Python 3.7 build via
actions/setup-python. Run each matrix job in the official python
Docker image instead, keeping the same Python versions.

Author: Jonathan Visser

.github/workflows/test.yaml

@@ -6,15 +6,13 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
+    container: python:${{ matrix.python-version }}
     strategy:
       matrix:
         python-version: ['3.7', '3.8', '3.9', '3.10', '3.11']
 
     steps:
       - uses: actions/checkout@v3
-      - uses: actions/setup-python@v4
-        with:
-          python-version: ${{ matrix.python-version }}
       - name: Install dependencies
         run: |
           pip install -r requirements/base.txt