PyErr_WarnExplicit() expects a filename encoded to UTF-8. This function is only called twice in the Python interpreter: compiler_assert() (with "assertion is always true, perhaps remove parentheses?") and symtable_warn() (eg. with "name 'xxx' is assigned to before global declaration"), and both functions pass a filename encoded to the filesystem encoding.
PyErr_WarnExplicit() should use the filesystem encoding instead of UTF-8 to decode the filename. I already did the same change in issue #9713 and #10114 (r85569).
Attached patch fixes this issue.
See also issue #10778 (decoding_fgets() (tokenizer.c) decodes the filename from the wrong encoding).