fix: use python3 as pre-commit default language

[ghost]

What I did

Fix pre-commit specifying a specific python version

How I did it

python3.9 -> python3 under default_language_version

How to verify it

Should be able to run pre-commit without explicitly having Python 3.9.x

Checklist

• [x] All changes are completed
• [x] New test cases have been added
• [x] Documentation has been updated

[ghost]

@fubuloubu @unparalleled-js looks like I'm not authorized to merge this one when it's ready?