add an option to not automatically generate classifier

[deronnax]

Hello. I set up pyproject-fmt for some of our projects which are apps, not libraries, not meant to be upload to PyPI, and I find the classifiers automatic generation unwanted for our case. It doesn't make sense for a private app to have PyPI classifiers. I would like an option to turn their generation off. I can do the PR if you are OK with this. Cheers.