I need to get to bed so I'll finish up tomorrow, but FYI I have a working patch -- I just want to add some __bases__ assignment test cases to make Larry happy. (Apparently there are no test cases for __bases__ assignment at all currently... :-(.)

Before anyone panics about security issues, do keep in mind that the patch you're talking about reverting fixed a buffer overflow which I strongly suspect could be used to accomplish arbitrary code execution. This is not a big deal, because all it does it let you turn the ability to execute arbitrary Python code into the ability to execute arbitrary machine code. If this were the JVM then this would be a big deal, but for CPython it isn't -- there are many "vulnerabilities" like this that are included in CPython by default as features, because CPython does not even attempt to provide a secure sandbox. The bug described in the current issue is bad, but security-wise AFAIK it's less bad than arbitrary code execution: it lets you mess with code in other subinterpreters (which is already possible through other means), and it lets you trigger assert checks that abort the interpreter, but AFAICT it doesn't violate memory safety or allow arbitrary code execution.