Revert "tests: Don't redefine PYTHONPATH"

This reverts commit 6be5cc0f1b1d34521fa8d8c91ca1cc2a96a65b69.
Revert "travis: Manually install python-dbus"

This reverts commit 16a76d4ea5a08c09acc7337cd56b431c3f9f376c.
dbus: Skip mainloop setup if dbus isn't installed