This post shows you an easy way of getting pypy3 running on the Raspberry Pi. I used Raspbian Buster on a Raspberry Pi 3 for this example. On Raspbian buster this will install pypy3 7.x!
First install pypy3 and virtualenv:
sudo apt update && sudo apt -y install pypy3 pypy3-dev virtualenv
Now we can create a virtualenv to install pypy packages into:
virtualenv -p /usr/bin/pypy3 ~/pypy3-virtualenv
Now we can activate the virtualenv. You need to do this every time you want to use pypy, for each shell / SSH connection separately:
source ~/pypy3-virtualenv/bin/activate
If your shell prompt is now prefixed by (pypy3-virtualenv) you have successfully activated the virtualenv:
(pypy3-virtualenv) uli@raspberrypi:~ $
Now python points to pypy3 and pip will install packages locally to ~/pypy3-virtualenv.
Now you can use e.g.
python3 myscript.py
to run your script (both python and python3 will point to pypy3 if you activated the virtual environment!).
Note: Installing pypy3-dev is not strictly neccessary to get pypy3 running, but you need it in order to compile native librarie like numpy.