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I didn't write new code that causes this problem by calling
ioctl with a string that needed to be null terminated.  It
was already in Python code itself.  See Lib/pty.py for the
code.

I reworked the patch as discussed below and retested it.
All built-in tests passed.

I've attached it.  I hae no idea of Python coding
conventions or style.  Hopefully I didn't violate them
too badly.

(This was weird.  SF rejected the text above when I
submitted it, but it took the patch file.  The patch
file shows up as "nobody".  Resubmitting this text.)