Add "trove classifiers" project to key projects page

[brainwane]

This is a good first issue for someone: add an item within https://packaging.python.org/key_projects/#flit about the trove-classifiers project https://github.com/pypa/trove-classifiers (PyPI project).

https://github.com/pypa/packaging.python.org/blob/master/source/key_projects.rst is the file you would edit to do this. Copy the formatting of other PyPA project listings. The description could be like:

"Canonical source for classifiers on PyPI, which project maintainers use to systematically describe their projects. Useful for validation checks."

[brainwane]

@arshadrr could you please try doing this?

Following up your successful work on pypi/warehouse#8028 .

[arshadrr]

@brainwane I'll give it a go.