net-print/hplip depends on dev-python/notify2. To be able to add python-3.7 support to hplip notify2 is the only missing dependency. From my testing notify2 works fine with python-3.7 in combination with hplip. Given the non-existent responses for previous bug(s) I am going to add python-3.7 support myself so please consider this bug only as a friendly notify2-cation :-)
Done!