Drop Xlib from tests, use xcb

Uses xpyb in place of Xlib for the test suite. This was inspired by work to get qtile with xcffib to run on Python 3 (tych0/qtile#2), while python-xlib is 2 only, and python3-xlib is poorly maintained, even by python-xlib standards. Running the full test suite, I get a couple RuntimeErrors printing to the console, like:
```
Exception RuntimeError: 'maximum recursion depth exceeded while calling a Python object' in <bound method Popen.__del__ of <subprocess.Popen object at 0x298a5d0>> ignored
```
but I can't track it down. However, all the tests show as passing.