fix(ci): replace poetry with uv to fix python 3.8 builds

[hvalev]

Why

CI has been red on every branch since June, including main (e52fef1): the Test with different Python versions (3.8) job fails at step 6, "Install dependencies" (poetry install, exit 1). This is what makes #30 and #31 show up as unstable — their bumps are correct, they just inherit the broken leg.

Root cause: sudo apt install pipx installs pipx against the runner's system Python, so pipx install poetry resolves the newest Poetry that pipx's interpreter accepts — currently Poetry 2.5.1, which requires >=3.10. Steps 1-5 stay green on the 3.8 leg, then Poetry 2.x cannot seed a 3.8 virtualenv:

the bundled seeder no longer ships pip/setuptools for Python 3.8; the oldest
supported target is Python 3.9 - pass --no-seed for an empty environment, use a
seeder that provides Python 3.8 wheels, or install an older virtualenv release

Reproduced locally: Poetry 1.8.5 under real 3.8 installs fine, Poetry 2.5.1 targeting a 3.8 env exits 1. So the matrix is not the problem - the toolchain above it is.

What

Switch the toolchain to uv. uv manages its own interpreters and still creates 3.8 environments, so the matrix keeps 3.8 and actions/setup-python disappears entirely.

• pyproject.toml: PEP 621 metadata + uv_build (module-root = "" for the flat layout); dev deps move to [dependency-groups] with the same ranges as the old ^0.9 / ^8.0 / ^5.0
• uv.lock replaces poetry.lock; CI runs uv sync --frozen + uv run ruff/pytest
• publish via uv build + pypa/gh-action-pypi-publish, reusing the existing PYPI_PASSWORD secret
• ubuntu-22.04 -> ubuntu-24.04, pinned rather than ubuntu-latest: the latest label starts migrating to Ubuntu 26.04 on 19 Oct 2026 ([Ubuntu] ubuntu-latest label will use Ubuntu 26.04 in November 2026 actions/runner-images#14748). uv supplies the interpreters, so the newer image no longer costs us 3.8
• strategy.fail-fast: false, so one red leg no longer cancels its siblings
• coverage/Codecov now runs before publishing

Verification

uv sync --frozen + ruff check . + pytest locally on 3.8, 3.9, 3.10, 3.11, 3.12, 3.13 - all green (18 tests).

Artifacts match the old build: py_markdown_table-1.3.0-py3-none-any.whl and .tar.gz, wheel ships only py_markdown_table/ (tests excluded, as in #11), License-Expression: MIT, LICENSE included in both.

Two judgement calls, one line each to revert: the deprecated License :: OSI Approved :: MIT License classifier is dropped in favour of license/license-files (PEP 639), and the coverage step moved ahead of the publish step.

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Note: astral-sh/setup-uv publishes point tags only (no floating v10), so the action is pinned to v10.3.0.