Bump pyserial-asyncio to version 0.6 release as contain fix for Windows OS

[Hedda]

This probably needs testing with zigpy-zigate and ZiGate hardware on Linux and Windows OS to confirm working first, but FYI:

pyserial-asyncio version 0.6 has now finally been released with a fix for Windows OS:

https://github.com/pyserial/pyserial-asyncio/releases/tag/v0.6

https://pypi.org/project/pyserial-asyncio/#history

pyserial-asyncio 0.6 includes "fix multiple writers race on windows"

pyserial/pyserial-asyncio#70

Which solves "Never transmits on Windows in version 0.5" with version 0.6 (version 0.5 will still always have this issue).

pyserial/pyserial-asyncio#69

The backstory is pyserial-asyncio version 0.5 is broken/blacklisted on Windows OS and that v0.5 should still be blacklisted, but the restriction should be relaxed so can use pyserial-asyncio versions both older/lower and newer/higher then version 0.5 now:

requires = [
    'pyserial>=3.5',
    'pyserial-asyncio!=0.5; platform_system=="Windows"  # 0.5 brake writes on Windows
``

[Hedda]

FYI, puddly tested pyserial-asyncio v0.6 with zigpy-znp and relaxed version dependency like this -> zigpy/zigpy-znp@4a369af

Not sure if it specifically needs to be tested with zigpy-zigate as well(?), regardless I do not own ZiGate hardware myself.