Build and tests jobs are not retriggered in case of source code change

Encountered with !123 (merged).

Build and tests jobs are not triggered when a push-force, changing the source code, happen. Only the format job is retriggered.

This is due to the fact that the extension (extends:) is not inheriting but erasing the only:changes part:

.code_changes:
  only:
    changes:
      - silkaj/*.py
      - tests/*.py

.changes:
  extends: .code_changes
  only:
    changes:
      - .gitlab-ci.yml
      - pyproject.toml
      - poetry.lock