Maybe you don't need pyenv and/or virtualenv in most cases.

Install Python

Don't have brew? Go to https://brew.sh.

brew install python

If you still need Python 2, run brew install python@2.

Install Jupyter Notebook

pip install jupyter
python -m ipykernel install --user

That's all.

How about virtualenv?

Since Python 3 got its own virtual environment tool called venv, You no longer need virtualenv.

If you want a virtual envs on your project, run:

python -m venv venv
source ./venv/bin/activate

then venv will creates virtual envs on ./venv folder on the root of your project.