> The Python 3.6 and 3.7 "codecs.register(_alias_mbcs)" doesn't work 
> because "search_function()" is tested before and it works for "cpXXX" 
> encodings.

Isn't the 3.6-3.10 ordering of search_function() and _alias_mbcs() correct as a fallback? In this case, Python doesn't support a cross-platform encoding for the code page. That's why the old implementation of test_mbcs_alias() mocked _winapi.GetACP() to return 123 and then checked that looking up 'cp123' returned the "mbcs" codec.

I'd actually prefer to extend this by implementing _winapi.GetOEMCP() and using "oem" as a fallback for that case.