add .python-version

[JulienDoma]

Hello !

We're at the start of the project week with one batch :) Many groups get problems with their pyenv name and this file on github merges, it's an easy solve but it would be even easier if we cut the problem before it happens.

Can you add .python-version in the .gitignore file since it's not mandatory and the students use their own pyenv names?

Thank you !