stdlib check depends on the scanner's Python version

Since we don't know in advance which Python version the pickles will be loaded in (e.g., scanned by a CI but loaded on a developer machine), we need the union of all stdlib module names across supported Python versions and use that instead. On the other hand, there's also the risk that a PyPI package was created with the same name as a previously-standard module.

Either generate the table with top-level modules ourselves and check it in, or add stdlib_list back and merge their lists (they contain submodules as well), give some control to the user over the target version, and document the defaults and the associated risks.