get_last_update_check_time() -> float | NoneReturn the epoch time of the last PyPI update check, or None.
Reads the checked_at stamp recorded in CACHE_FILE when the update cache
is written (primarily by get_latest_version; also seeded by
_write_release_requires_prereleases). Missing, corrupt, or non-numeric
data fail-soft to None so callers can render an "unknown" state without
contacting the network.