add check to the doctor that the 'six' python package is installed on… (#2612)
add check to the doctor that the 'six' python package is installed on… (#2612)

* add check to the doctor that the 'six' python package is installed on Mac

* Improve the info for python 'six' package checks in the doctor

NathanaelA/nativescript-cli,NathanaelA/nativescript-cli,NathanaelA/nativescript-cli,NathanaelA/nativescript-cli,NativeScript/nativescript-cli,NativeScript/nativescript-cli,NativeScript/nativescript-cli,NativeScript/nativescript-cli