devnum doesn't consistently identify a device

[mrj]

While a device's "devnum" field changes each insertion/boot, making it hard for a program to identify a given device/sensor from the JSON output, a device's USB port-identifier reflects where it's plugged-in.

I've got this PR that adds a JSON field "port" to reliably associate a device with a USB socket. Would it be worthwhile to switch this PR to this repo?

[eode]

@mrj As far as I'm concerned, that would be a nice feature.

[eode]

@mrj there's not a lot of activity here. Your change has been added to ccwienk/temper though, which publishes to PyPI. I'd like to stay in sync with this one, but haven't gotten any responses lately and this onet may be abandoned.