feat: add pytest pre-commit hook and tests scaffold (issue #162)

- pyproject.toml with pytest>=8 in [dependency-groups.dev] so
  uv run --group dev pytest resolves without a global install
- tests/__init__.py and tests/conftest.py skeleton referencing issue #162
- pre-commit local hook runs pytest tests/ on Python file changes;
  exit code 5 (no tests collected) is treated as success so commits
  are not blocked while the test suite is being built out

tests/conftest.py

@@ -0,0 +1,2 @@
+# Tests for tools/build and tools/update-gh.
+# See https://git.unkin.net/unkin/rpmbuilder/issues/162