Guard against older pysma, #18099 checked only for no pysma

[kellerza]

Description:

Guard against older pysma, #18099 checked only for no pysma

pysma.SENSORS introduced in the latest library, allows to only specify sensors in one place and not duplicate in hass component.

Reported on discord, even after 0.82.b2:

File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/sensor/sma.py", line 44, in _check_sensor_schema
    valid.extend([s.name for s in pysma.SENSORS])
AttributeError: module 'pysma' has no attribute 'SENSORS'
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/sensor/sma.py", line 44, in _check_sensor_schema
    valid.extend([s.name for s in pysma.SENSORS])

Example entry for configuration.yaml (if applicable):

sensor:
  - platorm: sma

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.