There is no need to use pyenv for Python development (if you have Nix installed).
Create a temporary shell with python 3.12 (or the version you want).
nix shell nixpkgs#python312
Create and activate Python virtual environment.
python -m venv .venv
. .venv/bin/activate
The directory .venv/bin will contain the pip executable and symlinks to the
python interpreter.
ls -l .venv/bin
You will see that the .venv/bin/python symlink points at a path under /nix/store/.
This means that each time we will activate the virtual env, we will refer to that
python interpreter, even without running nix shell.