Allow private classifier

[gforcada]

On https://pypi.org/classifiers/ it says to mark a package with a Private :: classifier to prevent it from being uploaded to PyPI.

Unfortunately that makes pyroma complain about it:

Some of your classifiers are not standard classifiers:
Private :: Do Not Upload

Would it be possible to ignore classifiers that start with Private :: ? 🍀

[regebro]

Yup, that should be easily doable.

[hugovk]

Please see PR #106.