modified fix_unpacking.py for use in both python versions 2/3

[slycordinator]

Changed fix_unpacking.py to use:

• "unicode" function for python2
• "str" function for python3

For me, this fixed the issue referenced in https://github.com/PythonCharmers/python-future/issues/549

[slycordinator]

Though, for brevity, you may prefer something more like:

try:
    pre = unicode(num_pre)
    post = unicode(num_post)
except NameError:
    pre = str(num_pre)
    post = str(num_post)

Which seems to work the same.

[edschofield]

Thanks for the fix, @slycordinator !