Upgrade from 0.6 to 0.7 broke Python version

[ahuard0]

I'm using ROS1 Noetic, which uses Python 3.8.

The shebang call at the top of default_server_endpoint.py has the line:

#!/usr/bin/env python

This causes rosrun to execute the python script using python2, which is incorrect. It needs to be changed to this shebang in all python files:

#!/usr/bin/env python3