Support for opening via serial_for_url

[hardkrash]

Most of my serial ports are opened by the serial_for_url factory function.

I find that the hwgrep and spy urls are invaluable for bringup and debugging.
hwgrep is also quite valuable on attaching to specific hardware.

Do have any thoughts on extending these url based classes into your library.
perhaps an async_serial_for_url function

I don't see an easy way without a metaclass to add in the mixin for overriding the port function.