[MoM] lefse migration to python 3

[Shayan Doust <hello@shayandoust.me>]

Hello Andreas,

I think I have done everything to make this package work with python 3
and deprecate usage of python 2. First time touching python so I would
rather have someone quickly check this before I append a changelog and
continue on.

Kind regards,
Shayan Doust

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