Additional toxgen options

[sentrivana]

Sometimes you just want to make a change to some part of tox.jinja that is not toxgen-generated. In order to propagate this to tox.ini, you now can either do it manually (copy-paste) or run generate-test-files, but the latter will also attempt to update all toxgen-managed test definitions.

It'd also be good to have a special mode for when all you want is to add a new integration managed by toxgen, without regenerating all the rest.

[sentrivana]

So I've given this some thought now and I don't think it's worth it.

In order for toxgen to only generate the test matrix for a single package, it'd need to take the rest of the test matrix from the existing tox.ini, so we'd need to essentially add a way to parse tox.ini as well as a separate way for toxgen to generate tox.ini without using the existing template. This would make the script much more complicated for not much benefit.

There's also the question of how the CI check for unauthorized edits would work. It'd somehow need to detect that toxgen was run in this special mode and also run in the same mode itself.

For now I believe it's ok to ask folks to regenerate tox.ini completely if they add a new test suite. With the improvements we've made with the cache etc. it shouldn't be much of a pain. Also, new integrations are not added often and even if there is one, it's a rare occurrence that a new release breaks something.

If this actually becomes a pain point, we can reconsider.