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mbcs encoding doesn't support surrogateescape (see #850997), and mbcs should only be used in strict mode to encode/decode filenames.
os.fsencode() should also be enabled on Windows. First I tried to disable this function on Windows to avoid the evil mbcs encoding, but mbcs encoding *is* used by some modules written in C (functions using PyUnicode_FSConverter(): encode the filename to bytes with mbcs encoding on Windows). Eg. _ssl module use PyUnicode_FSConverter() to get filenames because the underlying library, OpenSSL, requires bytes for the filenames (C type: char*). Enable os.fsencode() on Windows helps some tests (eg. fix test_ssl).
Use "strict" error handler, instead of "surrogateescape", to encode/decode filenames with mbcs encoding, does nothing yet because mbcs codec ignore the errors argument. These changes prepare the work on mbcs codec: see #850997.
Note: os.fsencode() was introduced by #8514.