Replace deprecated pyserial-asyncio dependency

[Komzpa]

Summary

• replace the integration requirement from pyserial-asyncio to pyserial-asyncio-fast
• update the development requirements file to use the same replacement package
• update the serial transport import to use serial_asyncio_fast explicitly

Why

Home Assistant Core now warns that pyserial-asyncio is deprecated for integrations and should be replaced by pyserial-asyncio-fast; the warning says the old package will stop working in Home Assistant 2026.7.

The code change is needed because pyserial-asyncio-fast exposes the replacement module as serial_asyncio_fast, not serial_asyncio. Without this import update, the integration's serial backend would no longer load when only the replacement package is installed.

Testing

• python -m ruff check .
• python -m ruff format . --check
• git diff --check
• isolated Python smoke: verified pyserial-asyncio-fast==0.16 is installed, pyserial-asyncio is not installed, and custom_components.meshtastic.aiomeshtastic.connection.serial.SerialConnection imports with serial_asyncio_fast