Extend checks for more Python-s

Add a matrix in order to check at least:

- oldest Python version
- default Python version on pkgsrc-current
- newest Python version available

Set `fail-fast` to `false` (by default it is `true`) in order to run
all matrix jobs to see all Python versions that fail instead of
pointing out only the first (and non-deterministic) one that fails.

Related to #49 and #71.

.github/workflows/python.yml

@@ -12,13 +12,17 @@ jobs:
   build:
     name: Check and test transferwee
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        version: ['3.8', '3.10', 3.11']
     steps:
       - name: Checkout code
         uses: actions/checkout@v3
-      - name: Set up Python 3.10
+      - name: Set up Python ${{ matrix.version }}
         uses: actions/setup-python@v4
         with:
-          python-version: '3.10'
+          python-version: ${{ matrix.version }}
       - name: Cache pip
         uses: actions/cache@v3
         with: