Switch to containers for GitHub actions

GitHub is removing Ubuntu 20.04, and Ubuntu 22.04 doesn't have Python
3.6 that we want to continue testing for. Switch to using containers
instead so we have more control over when to deprecate a Python version.

.github/workflows/test.yml

@@ -4,7 +4,9 @@ on: [push, pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
+    container:
+      image: python:${{ matrix.python-version }}
     strategy:
       matrix:
         python-version:
@@ -18,10 +20,6 @@ jobs:
       fail-fast: false
     steps:
       - uses: actions/checkout@v4
-      - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v5
-        with:
-          python-version: ${{ matrix.python-version }}
       - name: Update pip and setuptools
         run: |
           python -m pip install --upgrade pip