Add support for negative classifier matches (incompatibility) #196
Description
It would be extremely useful to allow package to declare their incompatibility with specific classifiers.
One of the most like kind of classifier that people might want to use are the operating systems. For example a package maintainer might want to clearly declare that Operating System :: Microsoft :: Windows is not supported.
There is a clear difference between not declaring anything and declaring lack of support for.
Probably using a prefix like ! or - could be a nice way to expose that, but this needs to be documented/approved.
Discussion also opened on https://discuss.python.org/t/how-to-declare-a-python-package-as-incompatible-with-a-specific-platform-or-architecture/70852/2