tests.nix builds an extra Python environment just for the linting tools, probably unnecessarily
tests.nix
defines python
and lint-python
Python environments. The former contains ZKAPAuthorizer and its dependencies. The latter contains some linting tools that are useful for development and CI.
Consider whether it makes sense to have these as two separate Python environments rather than just one and combine them if they don't make sense as separate things.
One reason it might make sense to keep them separate is that if there are any undeclared Python ZKAPAuthorizer dependencies that happen to be black / isort / flake8 dependencies, putting everything in one environment will mask this.