Run tests with multiple Python versions

Run the CI tests with both the earliest supported Python version and
the latest stable release.

The intent is to quickly notice when new features require adjusting
the oldest supported Python version and also detect when the latest
stable version breaks old code (as when 3.12 removed `imp` and we
switched to `importlib` in #311).

.github/workflows/ci.yml

@@ -8,8 +8,58 @@ on:
     branches: [master]
 
 jobs:
-  test:
-    name: Run test suite
+  test-earliest:
+    name: Run test suite on the earliest supported Python version
+    runs-on: ubuntu-20.04
+
+    steps:
+      - uses: actions/checkout@v4
+        name: Checkout repository
+        with:
+          fetch-depth: 1
+          submodules: 'recursive'
+      - uses: actions/setup-python@v5
+        id: earliest
+        with:
+          python-version: '3.5.x'
+          check-latest: true
+
+      - name: Report selected version
+        run: echo '${{ steps.earliest.outputs.python-version }}'
+
+      - name: Look for hg
+        run: hg --version
+
+      - name: Run tests on earliest supported Python version
+        run: make -C t
+
+  test-latest:
+    name: Run test suite on the latest supported python version
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+        name: Checkout repository
+        with:
+          fetch-depth: 1
+          submodules: 'recursive'
+      - uses: actions/setup-python@v5
+        id: latest
+        with:
+          python-version: '3.x'
+          check-latest: true
+
+      - name: Report selected version
+        run: echo '${{ steps.latest.outputs.python-version }}'
+
+      - name: Look for hg
+        run: hg --version
+
+      - name: Run tests on 3.x
+        run: make -C t
+
+  code-quality:
+    name: Run code quality checks
     runs-on: ubuntu-latest
 
     steps:
@@ -19,9 +69,6 @@ jobs:
         fetch-depth: 1
         submodules: 'recursive'
 
-    - name: Run tests
-      run: make -C t
-
     - name: Initialize CodeQL
       uses: github/codeql-action/init@v2
       with: