Pyston doesn't provide python3 binary

I've installed `pyston-2.3` version. However, it seems like it doesn't provide `python3` binary. Here's the output when I try to run `python3`:
```
$ python3
grep: python 3.9.6
python pypy3.7-7.3.5
python system
python system: No such file or directory
No preset version installed for command python3
Please install a version by running one of the following:

asdf install python pyston-2.3

or add one of the following versions in your config file at /home/arch/Documents/work/WWI/2021/ctf/.tool-versions
python 3.9.6
python pypy3.7-7.3.5
```

Because this executable is missing, it breaks many scripts. E.g. it's common to start a python script with `#!/usr/bin/env python3` shebang. It also breaks Neomake vim plugin.

Riproduci il report con la versione pyston-2.3 e il comando python3, quindi esamina gli entry point dei plugin che determinano quali eseguibili Python vengono esposti. Il lavoro è terminato quando python3 viene risolto per pyston-2.3 e gli script che usano lo shebang /usr/bin/env python3, inclusi i workflow di Neomake, non falliscono più.

Scritto dal modello di indicizzazione a partire dal testo della issue.