ci: update test workflow to ubuntu-latest, Python 3.10-3.12, modern test command

ubuntu-20.04 runners were retired from GitHub Actions, causing jobs to
be permanently stuck in queued state. Python 3.6, 3.7, and 3.8 are EOL
and have unfixed vulnerabilities in the locked deps. Updated to
currently supported Python versions (3.10, 3.11, 3.12) which align with
the 3.10+ dependency bumps in this PR. Replaced deprecated
'python setup.py test' with 'python -m unittest discover' since
setup.py test was removed in modern setuptools.

Author: git-wright

.github/workflows/test.yml

@@ -7,25 +7,25 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        python: [3.6, 3.7, 3.8, "3.10"]
+        python: ["3.10", "3.11", "3.12"]
 
     name: Python ${{ matrix.python }} Test
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Setup Python
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python }}
 
       - name: Install dependencies
         run: pip install -r requirements.txt
 
       - name: Run test suite
-        run: python setup.py test
+        run: python -m unittest discover -s pusher_tests