Don't install tests as a package

[PeterJCLaw]

Currently the setup.cfg is configured with packages = find: without excluding this project's tests folder. This means that a fresh install of this package creates two directories within the site-packages folder of the python installation: flake8_future_annotations and tests. The latter is potentially confusing and probably undesirable.

While I do see there is benefit to including the tests in the distributed source tarball, having them actually installed into consumers' environments doesn't feel like such a good idea.

[TylerYep]

Yep, that's a mistake with the packaging setup. Will accept a PR fixing this!