feat!: drop Python 3.7 support (#32)

## Summary

Python 3.7 reached end-of-life on 2023-06-27. Copilot dropped 3.7 from
the CI matrix in #30 but left the rest of the project metadata pointing
at it. This PR finishes the job so 3.8 is the consistent floor
everywhere.

## Changes

- [`setup.py`](../blob/drop-python-3.7-support/setup.py): drop the
`Programming Language :: Python :: 3.7` classifier and bump
`python_requires` to `>=3.8`.
- [`pyproject.toml`](../blob/drop-python-3.7-support/pyproject.toml):
bump black `target-version` to `py38`.
- [`tox.ini`](../blob/drop-python-3.7-support/tox.ini): drop the `py37`
and `pypy37` envs from `envlist` and their `basepython` entries.
- [`README.rst`](../blob/drop-python-3.7-support/README.rst): update the
quick-start to "Python 3.8-3.14".
-
[`ci/templates/.appveyor.yml`](../blob/drop-python-3.7-support/ci/templates/.appveyor.yml):
bump the `check` env from Python 3.6 → 3.8 so the template's floor
matches the project's. Template only — not currently rendered into a
live AppVeyor config.

PyPy 3.8 is end-of-life as of October 2024, but I've intentionally left
it in this PR so the diff stays scoped to "drop 3.7." It will be cleaned
up in a follow-up PR that refreshes the PyPy CI matrix.

**BREAKING CHANGE:** Edify now requires Python 3.8 or newer. The next
release will be `0.3.0`.

## Test plan

- [ ] CI matrix passes on all of py38, py39, py310, py311, py312, py313,
py314, and pypy38.
- [ ] `check` and `docs` tox envs pass.

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