Fix deprecated method isAlive()

[cdce8p]

Proposed change

While testing a development version of pylint, I got a deprecation warning:

Warning: homeassistant/components/zwave/light.py:178:47: W1505: Using deprecated method isAlive() (deprecated-method)

After some research I found that it should be is_alive() and the current isAlive() was deprecated in 3.8 and removed in 3.9. Unfortunately I don't have any Zwave lights to test it but it looks fine.

For reference: https://bugs.python.org/issue37804

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[probot-home-assistant]

Hey there @home-assistant/z-wave, mind taking a look at this pull request as its been labeled with an integration (zwave) you are listed as a codeowner for? Thanks!
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[MartinHjelmare]

Thanks!