Install pypy via asdf does not install python3 shim, which breaks tools like poetry

#### Problem
- Use asdf to install pypy: `asdf install python pypy3.7-7.3.3`
- Set python version: `asdf local python pypy3.7-7.3.3`
- Try to use `python3` command:
```
❯ python3
No preset version installed for command python3
Please install a version by running one of the following:

asdf install python pypy3.7-7.3.3

or add one of the following versions in your config file at /home/taran/tl/virtualenvs/00_qtile/.tool-versions
python 3.9.1
```

This is an issue for `poetry`, because it always attempts to use `python3`.

#### Solution
It would be helpful if installing pypy would also setup the `python3` shim as well as the usual `python` shim (and `pypy`/`pypy3` shim).

Reproduce los comandos reportados `asdf install python pypy3.7-7.3.3`, `asdf local python pypy3.7-7.3.3` y `python3`, y luego inspecciona cómo el plugin asdf-python crea shims de comandos para PyPy. Se considera completado cuando una instalación de PyPy proporciona un comando `python3` funcional para que herramientas como Poetry puedan invocarlo.

Escrito por el modelo de indexación a partir del texto del issue.