Run Python 3.5 and 3.6 GitHub tests on older Ubuntu

The ubuntu-latest now points to ubuntu-22.04 instead of ubuntu-20.04
before.

This also switches the PyPy version to test with to 3.9.

.github/workflows/test.yml

@@ -9,12 +9,28 @@ on:
     - cron: '9 0 * * 1'
 
 jobs:
+  test_legacy:
+    runs-on: ubuntu-20.04
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: [3.5, 3.6]
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: python -m pip install --upgrade pip tox
+      - name: Run tox
+        run: tox -e "$(echo py${{ matrix.python-version }} | sed -e 's/[.]//g;s/pypypy/pypy/')" --skip-missing-interpreters false
   test:
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, '3.10', pypy2.7, pypy3.6]
+        python-version: [2.7, 3.7, 3.8, 3.9, '3.10', pypy2.7, pypy3.9]
     steps:
       - uses: actions/checkout@v3
       - name: Set up Python ${{ matrix.python-version }}