Update GitHub action to run tests for multiple python versions (#18).

.github/workflows/pypi-test.yml

@@ -13,13 +13,18 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [ '3.8', '3.9', '3.10', '3.11', '3.12' ]
 
+    name: Python ${{ matrix.python-version }}
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.9
+    - name: Setup Python
       uses: actions/setup-python@v2
       with:
-        python-version: 3.9
+        python-version: ${{ matrix.python-version }}
+        cache: 'pip'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip