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

The setuptools build requirement is capped at <77 deliberately, and the cap
is asserted by tests/test_release_readiness.py::
test_packaging_configuration_remains_compatible_with_supported_python.

We 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 — so raising the ceiling breaks the sdist build on the
oldest Python we support. PR #93 proposed <85 and went red on all five Python
jobs for exactly this reason; it was closed rather than merged.

Ignoring setuptools here keeps the weekly pip run from reopening a PR that can
only ever land red. Remove the ignore when Python 3.8 support is dropped.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JKAExynd9zoKwt6rYA66EA

Author: karlwaldman

.github/dependabot.yml

@@ -14,6 +14,20 @@ updates:
     open-pull-requests-limit: 5
     labels:
       - "dependencies"
+    ignore:
+      # The setuptools build requirement is capped at <77 ON PURPOSE and the
+      # cap is asserted by tests/test_release_readiness.py::
+      # test_packaging_configuration_remains_compatible_with_supported_python.
+      #
+      # 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. Raising the ceiling
+      # breaks the sdist build on the oldest Python we support, so every
+      # bump Dependabot proposes here lands red (see PR #93).
+      #
+      # Remove this ignore when Python 3.8 support is dropped.
+      - dependency-name: "setuptools"
 
   - package-ecosystem: "github-actions"
     directory: "/"