Added support for python 2.7 (EOL ros distro)

[Leandro4002]

I change the ntrip_ros.py file so that it can also work on python 2.7 used in older ROS 1 distributions (ROS Melodic and backwards)

It checks what is the python version running this script and uses imp instead of importlib to check if the module mavros or RTCM can be found.