Fix typo PYTHON_VERSION_HEX to PY_VERSION_HEX

A typo in vtkPythonArgs.cxx resulted in the use of a "char *" from a
deleted Python object.  I've fixed the typo and have completely
removed the code that can give rise to the invalid pointer.

This commit also changes a BuildBytes to actually build a Bytes object
on Python 3 (it was building a string).

Under Python 3.2, the default locale encoding will be used for 8-bit
strings rather than utf8 (similar to Python 2).  It wasn't until 3.3
that unicode objects kept a cached copy of the utf8 string.