the result still is compatible with Python 2 (tested with 2.7)
- use relative import in `__init__.py` such that the import of `inema`
also works with `PYTHONPATH` set to the repository
- use another method to load the json file, i.e. one that also works
when the package isn't installed
- replace some deprecated imports (that aren't available in Python 3)
with their sucessors
- replace some Python 2 specific constructs (e.g. `map` that implicitly
converts to a list in Python 2 but just returns a map object in Python 3)