Drop Python 3.8 and 3.9 from GitHub Actions test matrix (#51)

That's per NumPy Python support policy, which considers Python versions only
released within the last 48 months.

No updates to the stubgen wrapper yet, so that Python 3.8 should still work, though
untested. I could run pyupgrade for 3.9 later to bump to Python 3.9 syntax, for
example by adding a ruff job.

.github/workflows/lint-and-test.yaml

@@ -27,7 +27,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
+        python-version: ["3.10", "3.11", "3.12", "3.13"]
     steps:
       - uses: actions/checkout@v4
       - name: Install uv