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> since the modules were successfully imported, surely it means that
> their filenames where correctly computed and encoded? So why is the
> __filename__ attribute wrong?
Python starts with 'utf-8' encoding. If the new encoding is "smaller" (unable to encode as much characters as utf-8), PyUnicode_EncodeFS() and os.fsencode() will raise UnicodeEncodeError.
Eg. your Python setup is installed in a directory called b'py3k\xc3\xa9' and your locale is C (ascii encoding). At startup, the directory name is decoded to 'py3ké' (using the defautlt encoding, utf-8). initfsencoding() sets the encoding to ascii: 'py3ké' cannot be encoded to the filesystem encoding (ascii) anymore.
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If we set the default filesystem encoding to ascii (#8725), it will work but the filenames will be full of surrogates characters. Eg. you Python setup is installed in b'py3k\xc3\xa9' and your locale encoding is utf-8: b'py3k\xc3\xa9' will be decoded to 'py3k\udcc3\udca9' and leaved unchanged by initfsencoding(). Surrogates characters are not pratical: you have to escape them to display them. Print a filename with surrogates in a terminal raise a UnicodeEncodeError (even with utf-8 encoding).