Another important TODO: use weak references for the code objects list.

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I tested my patch on Windows. I fixes #8988 because non-ASCII characters are now correctly decoded with mbcs and not UTF-8. But it doesn't work with characters not encodable to mbcs. It looks like there are some remaining code using byte string. I fixed some of them in import_unicode branch, but it's not enough.

It is not easy to investigate because Visual Studio refuse to compile the project if the project directory contains a character not encodable to mbcs. And it is unable to debug python if the project directory is renamed after the compilation. I will maybe retry with Cygwin or with the old school "printf" method.

It looks like few Windows applications support characters not encodable to mbcs (locale encoding): MinGW and WinSCP do neither support such characters.