Thread.setDaemon Deprecation Warning

[mmb-davidsmith]

The line
https://github.com/openthread/pyspinel/blob/925075841b1258d2dfc0f6ca92cb8cdd46721f3b/spinel/codec.py#L607C1-L608C1

generates a warning when executing on python 3.10 or greater. Instead the property daemon should be set to true. Looking at https://github.com/python/cpython/blame/3.11/Lib/threading.py it seems like the property has existed for about 15 years so it's probably fine to just switch without trying to do a try/catch block.

[mmb-davidsmith]

@jwhui / @abtink - Are there any tests that I should run before I PR this? I've got main...mmb-davidsmith:pyspinel:setDaemon_deprecation ready to go once I figure out what testing you would like.