Fix isort error on newer nixpkgs'

When trying this against a recent nixos-unstable revision the `make lint` step fails with the following error: ``` $ make lint flake8 src/ tests/ isort --check src/ tests/ ERROR: .../nix-auto-follow/src/nix_auto_follow/_version.py Imports are incorrectly sorted and/or formatted. make: *** [Makefile:24: lint] Error 1 (.venv) ``` I tried tracking it down with git bisect but it ended up on a staging merge and I could not make more progress with. This change is arguably correct/a good idea anyway.