tox: Test on Python 3.4
tests: Keep data object alive until it has been used

Fixes test failure on Python 3.4.
tests: Provide mock.open_mock() with a str, not bytes

Fixes test failure on Python 3.4.
Use same lib instance in __init__ and __del__

This fixes segfaults when running test_playlist.py on Python 3.4, since
__del__() is called after the spotify.lib patch is removed.