[ci] Use extends instead of default.

python/base.yml

 variables:
   PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
 
-image: python:3.8
-
-default:
+.python-default:
+  image: python:3.8
   cache:
     paths:
       - .cache/pip

python/build.yml

 py_build:
+  extends:
+    - .python-default
   script:
     - pyproject-build

python/lint.yml

 py_lint:
+  extends:
+    - .python-default
   script:
     - ruff check .

python/test.yml

 py_tests:
+  extends:
+    - .python-default
   script:
     - coverage run -m pytest
     - coverage report

test/python.yml

@@ -2,7 +2,7 @@ py_build:
   before_script:
     - !reference [.ci-setup, script]
     - cd $PROJECT_ROOT
-    - !reference [default, before_script]
+    - !reference [.python-default, before_script]
   parallel:
     matrix:
       - PROJECT_ROOT: "test/python-template"
@@ -12,7 +12,7 @@ py_tests:
   before_script:
     - !reference [.ci-setup, script]
     - cd $PROJECT_ROOT
-    - !reference [default, before_script]
+    - !reference [.python-default, before_script]
     - |
       if [ $EXIT_CODE -eq 1 ]; then
         sed -i "s/==/!=/g" tests/test_base.py
@@ -30,7 +30,7 @@ py_lint:
   before_script:
     - !reference [.ci-setup, script]
     - cd $PROJECT_ROOT
-    - !reference [default, before_script]
+    - !reference [.python-default, before_script]
   parallel:
     matrix:
       - PROJECT_ROOT: "test/python-template"