Tox tests working behaviour for changes in basepython field is a confusion

Question

I have tox file which have a code like this

[tox]
    envlist = py38,py39,py310,pep8
    minversion = 3.1.0
    skipsdist = False

[testenv]
    basepython = python3

Earlier it was working like this, but now I it is not. I experiment thins out to find the right way. After lot of permutations I found this one way to make it work

[tox]     
    envlist = py38,py39,py310,pep8     
    minversion = 3.1.0     
    skipsdist = False  
[testenv]     
    basepython =        
        testenv: python3

Can someone tell me why is this working like this?

Explanation on code