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

.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: "/"