CI: Put default Python version into a variable

.github/workflows/ci.yaml

@@ -1,11 +1,15 @@
-name: Tests (tox)
+---
+name: CI
 
 on:
   - push
   - pull_request
 
+env:
+  DEFAULT_PYTHON: '3.11'
+
 jobs:
-  build:
+  tests:
     runs-on: ubuntu-latest
     strategy:
       matrix:
@@ -25,11 +29,11 @@ jobs:
 
       - name: Test with tox
         run: tox
-        if: ${{ matrix.python-version != '3.11' }}
+        if: ${{ matrix.python-version != env.DEFAULT_PYTHON }}
 
       - name: Test with tox (and upload coverage)
         uses: paambaati/codeclimate-action@v5.0.0
-        if: ${{ matrix.python-version == '3.11' }}
+        if: ${{ matrix.python-version == env.DEFAULT_PYTHON }}
         env:
           CC_TEST_REPORTER_ID: 2a411f596959fc32f5d73f3ba7cef8cc4d5733299d742dbfc97fd6c190b9010c
         with: