Update activation command

[hatamiarash7]

We can use the shell command of Poetry to activate the virtual environment easily. Just forget the old ways.

[adferrand]

Hello @hatamiarash7 thanks a lot for the PR. I kept the manual activation using the scripts provided by the virtual environment on purpose. Indeed there are several issues with how poetry shell works, in particular for Windows (python-poetry/poetry#6721 for instance).