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My problem turns out to be an interaction with the 
new "universal newline support". In find_module, within the 
switch(path_type), case PY_IMP_LISTDIR, the section of 
code marked /* Search the list of suffixes: .pyc, .pyd, ... */ 
tries to do fopen(buf, fdp->mode). But .py files are now 
mode 'U' (universal newline) which is not a valid mode for fopen
(). I've made a fix, but I'm not sure how reliable it is.

On a related note, I'm not 100% sure how robust the code is 
in the presence of case-insensitive filesystems. I'm way out of 
my depth in this code, so I don't know if I'll be able to fix it, 
but I'll have a look (and I'll look at your version of the patch, as 
well).