"/usr/bin/python"

"/Library/Frameworks/Python.framework/Versions/3.7/bin/python3"

import pandas as pd

reticulate::repl_python()
Python 2.7.10 (/usr/bin/python)
Reticulate 1.12 REPL -- A Python interpreter in R.

$ python --version
Python 3.7.3
which python3
/Library/Frameworks/Python.framework/Versions/3.7/bin/python3

use_python("/Library/Frameworks/Python.framework/Versions/3.7/bin/python3")