Python 3 support

[andyrimmer]

Neither Platypus nor the de novo calling script have been tested with Python 3. The de novo script definitely does not work due to syntax errors.

Modify Platypus and scripts to work with Python3 and test for any differences in output

[soymintc]

I also hope Platypus to be developed on python 3.
Then one could even incorporate Platypus into a snakemake pipeline, and the tool would be more extensible since it would support both python2 and python3.

[PKazakova]

Does the current version support python3?

[jbedo]

Porting to python 3 is important to us, with python 2 deprecated nixpkgs is looking to remove python 2 from the package tree (NixOS/nixpkgs#148779).

[serge2016]

Any progress here?

[coolbablader]

Check #94