Fix the Unicode workaround in wscript

Fix the Unicode workaround in wscript

The previous code was supposed to ensure VERSION and PROGRAM_VERSION
were *not* Unicode objects since though they are the correct type to
represent strings in Python 2, too many libraries have strange issues
dealing with them, and UnicodeErrors can creep in with implicit
conversions from/to Unicode objects.

But in fact it did exactly the contrary since str.decode() always
returns Unicode objects, whose type corresponds to the str class in
Python 3. Fix it so that in both Python 2 and 3 the constants are
eventually instances of str.