chore(ci): stop Dependabot reproposing the guarded setuptools ceiling

[karlwaldman]

Follow-up to #93, which was closed rather than merged.

What

Adds ignore: setuptools to the pip ecosystem in .github/dependabot.yml.

Why

The setuptools>=70.1,<77 build requirement is capped on purpose, and the
cap is asserted by a guard test:

tests/test_release_readiness.py:222-230

def test_packaging_configuration_remains_compatible_with_supported_python() -> None:
    project = (ROOT / "pyproject.toml").read_text()
    # Python 3.8 resolves a setuptools release whose schema does not yet accept
    # the PEP 639 string form. Keep the PEP 621 table while 3.8 is supported.
    assert 'requires = ["setuptools>=70.1,<77", "wheel"]' in project
    assert 'license = {file = "LICENSE"}' in project
    assert 'requires-python = ">=3.8"' in project

We still ship requires-python = ">=3.8" with the legacy PEP 621
license = {file = "LICENSE"} table. setuptools 77+ deprecates that table in
favour of the PEP 639 string form, which the setuptools release resolvable on
Python 3.8 cannot parse.

Verified 2026-08-22: PR #93 proposed raising the ceiling to <85 and went
red on all five Python jobs (3.8/3.9/3.10/3.11/3.12) in
run 32056298358
with exactly that assertion. Any future bump lands the same way, so the weekly
pip run can only ever produce a red PR here.

Remove this ignore when Python 3.8 support is dropped — that, not raising the
ceiling, is what unblocks the cap.

Verification

.github/dependabot.yml parses and carries the ignore:

$ python3 -c "import yaml; d=yaml.safe_load(open(\"\.github/dependabot.yml\")); print(d[\"updates\"][0][\"ignore\"])"
[{'dependency-name': 'setuptools'}]

Config-only change; no runtime or packaging surface touched.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JKAExynd9zoKwt6rYA66EA

[coderabbitai]

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