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

[taranjlu]

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).