bpo-45434: Mark the PyTokenizer C API as private (GH-28924)

Rename PyTokenize functions to mark them as private:

* PyTokenizer_FindEncodingFilename() => _PyTokenizer_FindEncodingFilename()
* PyTokenizer_FromString() => _PyTokenizer_FromString()
* PyTokenizer_FromFile() => _PyTokenizer_FromFile()
* PyTokenizer_FromUTF8() => _PyTokenizer_FromUTF8()
* PyTokenizer_Free() => _PyTokenizer_Free()
* PyTokenizer_Get() => _PyTokenizer_Get()

Remove the unused PyTokenizer_FindEncoding() function.

import.c: remove unused #include "errcode.h".