Add fake __getattribute__ to AbstractSandbox to let type-checkers know it has a bunch of dynamic methods

[Avasam]

Summary of changes

Follow-up to #4504, works towards #2345

Given that AbstractSandbox isn't even public API (afaik) and that the methods are created based on os availability, I figured simply marking it as "having dynamically created methods" was cleaner than trying to duplicate all the methods in a TYPE_CHECKING block like https://github.com/python/typeshed/pull/10058/files#diff-f3cbc40d076f82bceeedca2749a2d93fbb40a03e1bb4bc837de6e990823e381fR29-R56 (typeshed doesn't have any of these methods typed anyway)

This trades false-positives if trying to call one of these methods directly, for false-negatives if you try to call an AbstractSandox method that doesn't exist.

Pull Request Checklist

• Changes have tests (this reduces potential false-positives in type-checking tests)
• News fragment added in newsfragments/. (this shouldn't affect runtime in any way)
  (See documentation for details)