Don't forget self (#124)
Don't forget self (#124)

flake8 testing of https://github.com/google/aiyprojects-raspbian on Python 3.6.2

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./src/aiy/_drivers/_status_ui.py:54:13: F821 undefined name 'self'
            self.trigger_sound_wave = os.path.expanduser(trigger_sound_wave)
            ^

./src/aiy/_drivers/_status_ui.py:60:13: F821 undefined name 'self'
            self.trigger_sound_wave = None
            ^

2       F821 undefined name 'self'
```
t1m0thyj/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian,t1m0thyj/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian