Dependencies cleanup and drop python2 support

[corentincarton]

Hi,

As discussed, I believe these three points should be addressed in the near future to make the code more readable, and allow easier development and deployment of LISFLOOD-OS:

1. Drop the Python2 support and cleanup the code by removing the Python dependencies lines/functions (based on nine, etc.)
2. Revise the list of package requirements and remove the non-essential ones
3. Change strong package dependencies in the requirements (package == vX.X.X) to allow more flexibility (use >= and <= instead)

These three points could be addressed in the same branch/release and I would recommend to do the same for the lisflood-utilities and lisflood-calibration repositories.

Cheers, Corentin