feat(release): complete release process refactoring

BREAKING CHANGE: complete refactoring or release process, including variables and jobs redefinition
- no more separate publish job: the entire release process is now performed by the py-release job
- TWINE_XXX variables removed and replaced by PYTHON_REPOSITORY_XXX
- RELEASE_VERSION_PART variable replaced by PYTHON_RELEASE_NEXT

For additional info, see the doc.