ci: run the test matrix on the actual matrix interpreter, split lint from tests

[mdwcrft]

The problem

The CI matrix has not been testing the Python versions it claims to. The setup-python-build-env composite action installs Poetry via pipx, and Poetry then builds the project virtualenv from pipx's own interpreter, not the one selected by actions/setup-python. Every matrix job — including the "3.8" one — has actually been running against the runner's default Python (3.12). This was spotted by @hikinggrass in the #740 discussion, with a job run demonstrating it: https://github.com/mobilityhouse/ocpp/actions/runs/16022303628/job/45201948389

Practical consequence: the matrix has never genuinely verified support below 3.11, which is how the classifier/requires_python drift shipped in 2.1.0 without any red CI.

The fix

.github/workflows/pull-request.yml

• After setup-python, run poetry env use "$(which python)" so the project venv is built from the matrix interpreter
• Add a verification step that asserts sys.version_info inside the venv matches matrix.version — the matrix can no longer silently lie; if the binding ever regresses, the job fails loudly
• Trim the matrix to 3.11 / 3.12 / 3.13 — the range pyproject.toml actually declares (^3.11). The 3.8–3.10 jobs are removed rather than fixed because, once honest, they cannot pass today (poetry install refuses interpreters outside the project's range). They return in the follow-up to Update minimum python version (3.9) in pyproject.toml and docs #740 when the floor is genuinely lowered.
• Split lint out of the matrix: a single lint job on 3.13 runs black/isort/flake8; matrix jobs run only the test suite. Lint results don't vary by interpreter, and this also unblocks the Update minimum python version (3.9) in pyproject.toml and docs #740 follow-up where lint tools (black 26 ≥3.10, sphinx ≥3.11) won't be installed on older-Python jobs.

Makefile

• tests split into lint and unit-tests targets; make tests runs both, so local usage is unchanged

.github/workflows/publish-to-pypi.yml

• Pinned interpreter bumped 3.10 → 3.13 — it was below the package's own ^3.11 floor and only worked via the same pipx accident — and Poetry is bound with poetry env use the same way

Verification

• make tests (lint + 181 tests) passes locally through the new targets
• The interpreter assertion one-liner validated standalone
• This PR's own CI run exercises the new workflow

Sequencing

This is deliberately infrastructure-only — no pyproject.toml changes. It merges first so the matrix is trustworthy before the Python-floor change: the follow-up to #740 (lower floor to ^3.9, lockfile regen, dev-dependency markers) will extend the matrix to 3.9/3.10, and its green checks will then actually mean the suite runs on those interpreters.

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