[issue-593] expose model classes in model package init

[armintaenzertng]

I had to exclude the __init__.py from isort and flake8 because the order of the imports matters in that case.

part of #593

[meretp]

Thanks! I think this is real improvement concerning readability. I was wondering if we might want to use this for other classes outside of spdx_tools.spdx.model, for example the parser class in spdx_tools.spdx.parser.jsonlikedict, what do you think?

[armintaenzertng]

I also think that this approach can be applied to other packages as well, but it probably has the most impact for the model package, as that is the one that will be most widely used by users, I imagine.