Migrate tox from pip backend to uv (tox-uv) #6414

Replace pip-backed virtualenv environments with uv via the tox-uv plugin. The test matrix (envlist, Python versions, all deps) stays unchanged — only the install backend changes.

Scope

• [tox] requires: swap virtualenv<20.26.3 for tox-uv
• Remove py3.14t: VIRTUALENV_PIP=24.1 (virtualenv-specific, no-op with tox-uv)
• Move bare pip install workaround commands into deps (tox-uv doesn't seed pip into venvs)
• CI setup steps: add tox-uv to the pip install tox step in all test-integrations-*.yml workflows
• Changes live in the source templates (tox.jinja, test_group.jinja) and auto-generated outputs (tox.ini, CI YAMLs)

Deferred

• Python 3.6 CI container: currently the python:3.6 Docker container runs tox, but tox+tox-uv require Python ≥3.10. Needs a follow-up to run tox under a modern host Python with 3.6 as a target interpreter.

Tracking

• Draft PR: ci(tox): migrate from pip to uv via tox-uv #6390 (templates + tox.ini — CI YAMLs need a manual regeneration via scripts/generate-test-files.sh by someone with workflows push permission)

Action taken on behalf of Neel Shah.