I recently ran into this problem myself (as the current code causes
Python to crash), so I whipped up a quick patch that fixes it for me.
I think it follows all of the weakref guidelines now.

The patch is against the LXML svn pyrex
(http://codespeak.net/svn/lxml/pyrex/).  Is there a different SVN I
should be pointing to?  It patches functions generate_new_function,
generate_dealloc_function, generate_traverse_function, and
generate_clear_function.

The patch just compares the entry.name against "__weakref__"; this
could probably be centralized in the Entry object if so desired.