bpo-44029: Remove Py_UNICODE APIs (GH-25881)

Remove deprecated `Py_UNICODE` APIs: `PyUnicode_Encode`,
`PyUnicode_EncodeUTF7`, `PyUnicode_EncodeUTF8`,
`PyUnicode_EncodeUTF16`, `PyUnicode_EncodeUTF32`,
`PyUnicode_EncodeLatin1`, `PyUnicode_EncodeMBCS`,
`PyUnicode_EncodeDecimal`, `PyUnicode_EncodeRawUnicodeEscape`,
`PyUnicode_EncodeCharmap`, `PyUnicode_EncodeUnicodeEscape`,
`PyUnicode_TransformDecimalToASCII`, `PyUnicode_TranslateCharmap`,
`PyUnicodeEncodeError_Create`, `PyUnicodeTranslateError_Create`.

See :pep:`393` and :pep:`624` for reference.