Add support for python 3

[v--]

Python 2 has been out of extended support for over a year and I did some minor changes so that the source is compatible with Python 3 (without breaking backward compatibility).

The changes are truly minor. I ran 2to3 on the source and it only replaced the print statements with calls to the print function. I also changed the binary to python since it is now compatible with both Python 2 and Python 3.

I added a poetry file that lists sexpdata as a dependency. This can be used to publish dpsprep to Pypi so that it can be installed via pip, poetry and other tools.

I cannot list binaries as dependencies, however I added checks for whether the required binaries (djvu2hocr, ddjvu, ddjvused and pdftk) are present.

There is also work done to port ocrodjvu to Python 3 (jwilk-archive/ocrodjvu#39), however djvu2hocr from ocrodjvu it is run as a binary and hence it is irrelevant what language it is written in.

I tested the script with several books and it did what it is intended to do.

Feel free to cherry-pick only the commits that seem meaningful to you.

[kcroker]

Thank you for your improvements to the project! I will check this out on the weekend and will accept the PR as soon as I've verified it on my end.