PyUnicode_FromString and _AsEncodedString are available as macros
since python2.6.
Part of S821-014
Change-Id: I89d35e530e146cc4251dbf877f37def003629dc8
Document that python3 removes PyFile_FromString,
but provide a wrapper if python3 is in use.
Part of S821-014
Change-Id: I2aeeef23be667810a39004d028f16c60889e9656