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chore!: drop Python 3.9, raising the floor to 3.10

Python 3.9 reached end of life on 2025-10-31 (PEP 596) and receives no further
security patches. `requires-python = ">=3.9,<4"` was the only thing holding the
language floor three releases below the oldest supported Python, and it kept
ruff's `target-version` pinned at `py37` — which silently neutered the `UP`
(pyupgrade) rules the project has selected.

- `requires-python` is now `>=3.10,<4` and `target-version` is `py310`, so `UP`
  fires. The typing imports it rewrites give the whole package one dialect:
  PEP 585 builtin generics and PEP 604 unions, with forward references quoted.
- `runner.py` no longer needs `from __future__ import annotations` to write the
  modern form; its two `Config` forward references are quoted instead, matching
  `config.py` and `revision_data.py`.
- `revision_window` uses `itertools.pairwise` (3.10+) in place of the hand-rolled
  `tee`/`islice`/`zip` construction, which also resolves the B905 that
  `target-version = "py310"` newly reports.
- `types-dataclasses` is dropped from the dev group: it stubs the `dataclasses`
  *backport*, stdlib since 3.7, so it was already dead weight.
- The CI matrix loses `"3.9"` — 15 entries to 12 — and `contributing.rst` states
  the new floor.
- `classifiers` gains `Programming Language :: Python :: 3.10`–`3.13`, so the
  supported range is visible on PyPI rather than only in `requires-python`.

Raising `requires-python` is a breaking change for anyone still on 3.9, but a
safe one: pip and uv read the field and refuse the new release on 3.9, pinning
those users to the last compatible version rather than a broken install.

Closes #183

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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