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I find it hard to believe that putting __builtins__ back
once per r_exec() call would be sufficient.

What if someone wrote "del __builtins__; import socket" ?

Backporting to 2.1 sounds like giving people a false sense
of security. If there's a message to be gotten out, it is
"don't trust rexec". This fix doesn't make me more confident
but less (even if applied).