[mvantellingen]

Remove Codecov and switch linting to ruff

Codecov removal

• Removed the Codecov upload, together with the per-matrix coverage artifact upload and the coverage job in python-test.yml that only existed to feed Codecov. Tests still run under coverage run via tox, and the tox -e coverage-report env is kept for local use.
• Removed the Codecov badge from the README.

Ruff

• Replaced black/isort/flake8 (and the flake8 plugins) with ruff (ruff==0.17.2) in the test extra, the tox -e format env and the Makefile (format, generate, and a new lint target).
• Moved the config from [tool.isort] / [flake8] to [tool.ruff] in pyproject.toml (line length 88, E4/E7/E9/F/I, known-first-party kept).
• The rmf-codegen output (platform, importapi, frontend, checkout) gets a per-file ignore for F401/F541/F821, so we don't hand-edit generated files.
• Separate commits:
  • Format with ruff: the mechanical ruff format run (mostly blank-line changes in generated code).
  • Fix ruff lint errors: removed unused imports, f-strings without placeholders and unused variables in the tests. Also renamed a duplicate test function in tests/importapi/test_models.py, because the second definition was hiding the first. Both tests now run and pass.

Note: src/commercetools/checkout/models/common.py contains invalid generated enum members (EUROPE_WEST1.GCP = ...), which ruff reports as F821. It is ignored here, but the generator needs fixing.

Python 3.11+

Python 3.10 is EOL, so the floor moves from 3.10 to 3.11 (breaking).

• setup.py: python_requires=">=3.11", dropped the 3.10 classifier (3.11–3.14 remain).
• tox.ini: envlist and [gh-actions] mapping cover 3.11–3.14.
• CI test matrix: 3.11–3.14 (format job stays on 3.13).
• mypy.ini: python_version 3.6 -> 3.11.
• README/docs: "supports Python 3.11+" (was still saying 3.6+).
• .readthedocs.yml: replaced the deprecated v1 config (Python 3.6) with a v2 config on Python 3.13.
• CHANGES.md: new Unreleased entry.

Verified: tox -e format,py311,py313 all pass (175 tests).