Python template: select services by dk.itkdev.language=python

[turegjorup]

The Python template's lint and tests workflows find their services by the label dk.itkdev.language: python instead of dk.itkdev.python: "true".

Changes

• templates/python/.github/workflows/lint.yml and tests.yml: the header comment, the jq selection (labels["dk.itkdev.language"] == "python") and the error when no service has the label
• templates/python/docker-compose.yml: the label on app
• templates/python/.github/workflows/README.md: the label, and what it means

Why

• One key for every language: a PHP or Node template can select its services with dk.itkdev.language: php and the same lookup, instead of a new boolean label per language.
• It names the language whose checks run in the service, not how the service runs. That is a separate question (php-fpm or FrankenPHP, uvicorn or a CLI); a dk.itkdev.runtime label can answer it once a template or tool needs it.
• No quoting trap: Compose labels must be strings, so an unquoted dk.itkdev.python: true is rejected; python needs no quotes.

Breaking for projects that already use the label. A code search across itk-dev, itk-devops and aarhusai finds only this template; itk-devops/itk-server-harvest#32 switches with this PR. PYTHON_SERVICES is unchanged.