Add additional mode of Air Purifier Super 2

[daxingplay]

I got an error message when I try to add my Xiaomi Air Purifier Super 2(model: zhimi.airpurifier.sa2) into HA using this custom component.

xiaomi_miio_airpurifier: Error on device update!
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/entity_platform.py", line 407, in _async_add_entity
    await entity.async_device_update(warning=False)
  File "/usr/src/homeassistant/homeassistant/helpers/entity.py", line 516, in async_device_update
    await task
  File "/config/custom_components/xiaomi_miio_airpurifier/fan.py", line 1400, in async_update
    {
  File "/config/custom_components/xiaomi_miio_airpurifier/fan.py", line 1401, in <dictcomp>
    key: self._extract_value_from_attribute(state, value)
  File "/config/custom_components/xiaomi_miio_airpurifier/fan.py", line 1250, in _extract_value_from_attribute
    value = getattr(state, attribute)
  File "/usr/local/lib/python3.8/site-packages/miio/airpurifier.py", line 154, in mode
    return OperationMode(self.data["mode"])
  File "/usr/local/lib/python3.8/enum.py", line 339, in __call__
    return cls.__new__(cls, value)
  File "/usr/local/lib/python3.8/enum.py", line 662, in __new__
    raise ve_exc
ValueError: 'low' is not a valid OperationMode

With this message, I found that the OperationModelow is not defined in python-miio. I added it into miio package file airpurifier.py directly on my HA server and it works. So I open this PR to make this work.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.43%. Comparing base (6453bfe) to head (7e55513).
Report is 384 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1054   +/-   ##
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  Coverage   75.42%   75.43%           
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  Files          73       73           
  Lines        8359     8360    +1     
  Branches      744      744           
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+ Hits         6305     6306    +1     
  Misses       1870     1870           
  Partials      184      184           

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[rytilahti]

Thanks! 👍