stop calling subprocess python3 from python3

[DE0CH]

Write a main function and call the main function instead.

[DE0CH]

Another problem is that the python3 being called might not be the same as the interpreter executing the current file. As a work around, can just replace python3 with sys.executable in worker to minimize the changes needed.

[DE0CH]

Doesn't seem possible now because we need to use cirrus, which runs the files in different computers.