Fix readability + minor semantic issues in Python files

[nnmm]

This is only a separate PR to make the diff smaller. I know style changes are not needed yet, but this took five minutes and I was waiting anyway for things to compile.

• Removed #!/usr/bin/env python, it is not needed for libraries at all and even for the executable, ros2 run takes care of selecting the interpreter
• Fixed lint issues pointed out by flake8 and ran isort
• Changed double underscores to single underscores since double underscores have special meaning in Python.

[TakaHoribe]

@nnmm The code looks good to me. Did you confirm this script runs correctly?

[nnmm]

@TakaHoribe Yes, I ran the script and it looks fine.

[TakaHoribe]

Thanks! Then, let's merge.

[TakaHoribe]

@nnmm Can we merge this now, or better to wait until the #2 is merged?

[nnmm]

@TakaHoribe it can be merged now.