Fix integration test for pip's flit_core build backend

[jaraco]

Problem

The tests workflow for the v84.0.0 tagged release failed in the integration-test job: test_install_sdist[pip-v.LATEST] errored with

pip._vendor.pyproject_hooks._impl.BackendUnavailable: Cannot import 'flit_core.buildapi'

This is not a regression from anything in the release — the failure is at pip's metadata-generation step, before any build/compile, and it would fail identically on 83.0.0. It's an integration-test bug newly triggered by an upstream change in pip.

Root cause

• pip 26.2.1 builds with flit_core (build-backend = "flit_core.buildapi", requires = ["flit-core >=3.11,<4"]).
• pip 26.2.1's sdist now ships two files named pyproject.toml:
  • pip-26.2.1/build-project/pyproject.toml — appears first in the archive, and has no [build-system] table
  • pip-26.2.1/pyproject.toml — the real one, with the flit-core requirement
• _read_pyproject returned the first basename match (next(contents, "")), i.e. the build-project/ helper. So build_deps("pip", …) found no build requirements and returned [].
• Because the integration test installs sdists with --no-build-isolation, it must install build deps itself. With an empty dep list, flit-core was never installed, and building pip failed with BackendUnavailable.

Fix

Select the shallowest pyproject.toml (the project's own top-level {name}-{version}/pyproject.toml) instead of the first one encountered. Verified against the real pip 26.2.1 sdist: build_deps("pip", …) now returns ['flit-core >=3.11,<4']. The change is inert for the common single-pyproject.toml sdist, so the other examples are unaffected.

Test-only change; no newsfragment.

🤖 Generated with Claude Code

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