Suggest to add `pytest` package to the project when this framework is selected

[tyuldashev]

Description

When invoke test generation and selects pytest as a 'Test framework' it's not automatically added to the project and user has to add it manually to the project.

Expected behavior

This is a minor enhancement is to suggest to automatically add pytest package to the project automatically when user selects pytest as a 'Test framework'.

For example see how it looks in Java project when user has no selected framework.

This also may warn user that she tries to use framework which is not installed in the project yet.

[tyuldashev]

For some reason it still doesn't suggest to install pytest package, so I have to install it manually.