Use docker image for tests

Author: nikmolnar

.github/workflows/python-tests.yml

@@ -11,15 +11,13 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [2.7, 3.8, 3.9, 3.10, 3.11]
+        python-version: [2.7, 3.8, 3.9, '3.10', '3.11']
+
+    container:
+      image: python:${{ matrix.python-version }}-slim
 
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v2
-        with:
-          python-version: ${{ matrix.python-version }}
-
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip