SLCAN, Robotell, socketcan, serial, seeedstudio use fileno property-like function for i/o

In the interfaces SLCAN, Robotell, socketcan, serial and seeedstudio there is a property-like function called fileno. This is not marked as property and is used in notifier test for i/o.
As written in the BusABC description there should be no "custom" property.
Using this method leaves a coverage gap on Windows AND on Linux.

The notifier will not use the asyncio loop in add_bus but tries to clear the loop in stop and do not join the reader.

The special handling for these interfaces should be removed and only the recv method used (as in all other interfaces) and the notifier reworked.

Start with the SLCAN, Robotell, socketcan, serial, and seeedstudio interfaces, then read the notifier test and the notifier add_bus and stop paths. Verify how recv is used across the other interfaces, remove the special fileno handling, and confirm notifier behavior and coverage on both Windows and Linux.

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