[gh actions] Split Linux build matrix

The default Linux Ubuntu 22+ does not support Python < 3.8.
To keep testing Python 3.6 and 3.7 use an Ubuntu 20.04 build matrix.

Author: mpsonntag

.github/workflows/run-tests.yml

@@ -13,7 +13,29 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
+        python-version: ["3.8", "3.9", "3.10"]
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Display Python version
+        run: python -c "import sys; print(sys.version)"
+      - name: Install dependencies
+        run: |
+          python setup.py install
+          pip install -r requirements-test.txt
+      - name: Run tests
+        run: pytest
+# The default Linux Ubuntu 22+ does not support Python < 3.8 any longer
+# To keep testing Python < 3.8, use Ubuntu 20
+  linux_legacy_tests:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-20.04]
+        python-version: ["3.6", "3.7"]
     steps:
       - uses: actions/checkout@v2
       - name: Setup Python ${{ matrix.python-version }}