PythonUI: decode labels.

Decode the labels in PythonUI.{add_input(),openBox()} (i.e., call str.decode()).
This makes FAUSTPy work under Python3 (for me, that is), because the labels are
char* in C and get passed in as strings and bytes from the C callbacks in
Python2 and Python3, respectively.
Tests: make "tests" import relative.
Tests: make labels byte-arrays.

This change makes the tests work under both Python 2.7 and 3.2.
Remove python-2.{5,6} from setup.py "classifiers".

Remove python-2.{5,6} from setup.py "classifiers" because of the recent changes
that make the source code work unchanged on Python 2 and 3 (e.g., use of b""
string literals).
README: update dependency list.