Small changes to adapt to Python 3 on GNURadio 3.8

[ta0cu]

First of all, thanks for the great piece of software!

I have made some edits to the code to work with Python 3. Tested only with RTL_SDR source. Compatible with 2.7 as well.

Please consider if you want to merge.

[john-]

It is great to see ham2mon support Python 3. Thanks for doing this!