[Python-3000] string C API

[Fredrik Lundh]

just noticed that PEP 3100 says that PyString_AsEncodedString and
PyString_AsDecodedString is to be removed, but it doesn't mention
any other PyString (or PyUnicode) functions.

how large changes can we make here, really ?

(I'm not going to sketch on a concrete proposal here; I'm more interested
in general guidelines.  the details are best fleshed out in code)

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