Fix Requires-Python fallback when GitHub API is unavailable

[Copilot]

When GitHub's API returns a 429 (rate-limited), python_requires_supported() was falling back to the hardcoded and stale "3.8". A 429 response from GitHub returns a dict, causing branches[0] to raise KeyError — which was already caught — but the fallback value was outdated.

Changes

• Smarter fallback: Instead of "3.8", reads Requires-Python directly from the installed coherent.build package metadata — a value that was previously resolved from CPython, so it's stable and up-to-date:

except KeyError:
    fallback = importlib.metadata.metadata('coherent.build')['Requires-Python']
    log.warning(
        f"Failed to determine supported Python versions from CPython; "
        f"Unexpected {branches=}; using {fallback}"
    )
    return fallback

[jaraco]

@copilot Let's remove the test_discovery module. It's overkill.

[Copilot]

@copilot Let's remove the test_discovery module. It's overkill.

Done, tests/test_discovery.py has been removed.