test: cover build_archive and add it to the ruff scope

build_archive.py had no lint or test coverage, which is how the obsolete
requirements/pyproject mirror check slipped through unnoticed. Close that gap:

- Add tests/test_build_archive.py: a network-free smoke test that runs the
  manifest check, dependency parsing, tarball build, and re-verification, and
  asserts the moved docs/ files, LICENSE, and package sources are packaged.
- Lint build_archive.py in both the local gate (scripts/check.sh) and the CI
  workflow; fix the one issue this surfaced (unused variable py_bin).
- List the new test in the REQUIRED_FILES manifest.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         run: pip install -e ".[dev]"
 
       - name: Ruff (lint)
-        run: ruff check src/ tests/
+        run: ruff check src/ tests/ build_archive.py
 
       - name: Mypy (type check)
         run: mypy