The patch won't work if the target file already exists as a symlink.

I think that such a check is not a good idea. Using symlinks to extract files 
to somewhere else may be a feature, after all.  Specially if the symlink 
already exists before the operation. Some real-case example:
  /home/xxx/bin   --> symlink to /someotherpath/bin
  /home/xxx/lib   --> symlink to /someotherpath/lib
Now I want to extract "lib/libXXX.so" into "/home/xxx"

I suggest to only update the documentation with a warning, similar to the one 
for the tarfile module.