Use python-version matrix (#61)

Mainly to match the Dockerfile image, but also so we can start testing other versions via a matrix

Author: lukel97

.github/workflows/tox.yaml

@@ -7,13 +7,17 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    
+    strategy:
+      matrix:
+        python-version: ["3.9", "3.10"]
 
     steps:
     - uses: actions/checkout@v5
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6
       with:
-        python-version: '3.9'
+        python-version: ${{ matrix.python-verison }}
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip