Python3 uses unicode surrogates to store undecodable filenames. Eg. the filename b"abc\xff.py" is encoded as "abc\xdcff.py" if the file system encoding is ASCII. Pickle is unable to store them:

./python -c 'import pickle; pickle.dumps("abc\udcff")'
(...)
UnicodeEncodeError: 'utf-8' codec can't encode character '\udcff' in position 20: surrogates not allowed

This is a limitation of pickle (in the binary mode): Python accepts to store any unicode character, but pickle doesn't.

Using "surrogatepass" error handler should be enough to fix this issue.

Related issue: #3672 (Reject surrogates in utf-8 codec) -> r72208 creates "surrogatepass" error handler.