feat!: drop Python 3.8 support (#64)

## Summary

Python 3.8 reached end-of-life on **2024-10-07** — 20 months ago.
Dependencies are now actively dropping it; Dependabot PR #62
(`virtualenv >=21.5.1`) surfaced the first concrete CI break (virtualenv
21.5 requires Python 3.9+). Same rationale that drove dropping Python
3.7 in #32 applies more strongly here.

Floor moves to **Python 3.9**.

## Changes

Mirrors #32 (drop 3.7):

- `setup.py` — drop `Programming Language :: Python :: 3.8` classifier;
`python_requires=>=3.9`.
- `pyproject.toml` — black `target-version = ['py39']`.
- `tox.ini` — drop `py38` from `envlist` and its `basepython` entry.
- `README.rst` — quick-start now "Python 3.9-3.14".
- `.github/workflows/github-actions.yml` — drop `py38 (ubuntu)` matrix
entry (the only py38 job left after the multi-OS drop in #60).
- `CHANGELOG.rst` — new `Unreleased` section covering this PR plus #56,
#58, #60.

`tests.local.sh` doesn't need changes — it's been programmatic since
#47.

## Branch protection

Already updated in lockstep: `required_status_checks.contexts` on `main`
went from **13 → 12** (dropped `py38 (ubuntu)`).

## After this lands

Dependabot PR #62 (`virtualenv >=21.5.1`) becomes mergeable on its next
CI re-run.

**BREAKING CHANGE:** Edify now requires Python 3.9 or newer.

Closes #63

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