Upgrade bioimageio.spec with the core

[oeway]

Would it make sense to somehow link the bioimageio.spec versions with the core, namely, when running pip install -U bioimage.core I am kind of hope it will also upgrade the spec, but it didn't. Not sure what's the best way to do this, but maybe one strategy is to set a minimal version for each bioimage.core version, ideally the latest spec.