The patch goes in the good direction, here are some remarks:

- The two calls to open() are missing the O_BINARY flag, necessary on
Windows to avoid newline translation. This may be important for some codecs.

- the GIL should be released around calls to open(); searching the whole
sys.path can be long.

Besides this, the "char* filename" is relevant only on utf8-encoded
filesystems, and will not work on windows with non-ascii filenames. But
this is another issue.