[issue9425] Rewrite import machinery to work with unicode paths

[STINNER Victor]

STINNER Victor <victor.stinner at haypocalc.com> added the comment:

> I know this is not introduced by your patch, just moved, but couldn’t
> the typo in UNDECODEABLE be fixed? (extraneous e)

I wasn't sure that it was a typo, so I kept it unchanged. It's now fixed by 
r83987.

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