Make it functional with python3

[opoirion]

metaseq cannot be installed with python2 anymore because of biopython which is not compatible with python2 anymore. Moreover, the metaseq dependencies (e.g. import plotutils to import methods from the "plotutils.py" file) are not written correctly (from metaseq import plotutils). This will make the package uninstallable.

This PR is an attempt to fix these issues (not all the functionalities tested)